Annual report pursuant to Section 13 and 15(d)

Derivative Instruments and Hedging Activities Schedule of Interest Rate Swaptions (Details)

v3.8.0.1
Derivative Instruments and Hedging Activities Schedule of Interest Rate Swaptions (Details) - USD ($)
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Derivative [Line Items]      
Notional $ 31,226,878,000 $ 18,802,500,000 $ 21,998,381,000
Interest Rate Swaption [Member] | Long [Member] | Variable Income Interest Rate [Member] | Less Than Six Months Remaining Maturity [Member]      
Derivative [Line Items]      
Cost Basis 21,380,000 29,360,000  
Fair Value $ 17,736,000 $ 42,149,000  
Weighted Average Remaining Maturity 4 months 1 day 1 month 7 days  
Interest Rate Swaption [Member] | Long [Member] | Variable Income Interest Rate [Member] | Six Months or Longer Remaining Maturity [Member]      
Derivative [Line Items]      
Cost Basis   $ 13,655,000  
Fair Value   $ 792,000  
Weighted Average Remaining Maturity   6 months 21 days  
Interest Rate Swaption [Member] | Long [Member] | Fixed Income Interest Rate [Member] | Less Than Six Months Remaining Maturity [Member]      
Derivative [Line Items]      
Cost Basis $ 4,660,000    
Fair Value $ 2,982,000    
Weighted Average Remaining Maturity 3 months 22 days    
Interest Rate Swaption [Member] | Short [Member] | Variable Income Interest Rate [Member] | Less Than Six Months Remaining Maturity [Member]      
Derivative [Line Items]      
Cost Basis $ 7,950,000 $ 51,355,000  
Fair Value $ (5,619,000) $ 1,414,000  
Weighted Average Remaining Maturity 4 months 20 days 5 months 25 days  
Interest Rate Swaption [Member] | Short [Member] | Variable Income Interest Rate [Member] | Six Months or Longer Remaining Maturity [Member]      
Derivative [Line Items]      
Cost Basis   $ 29,893,000  
Fair Value   $ 938,000  
Weighted Average Remaining Maturity   6 months 23 days  
Interest Rate Swaption [Member] | Short [Member] | Fixed Income Interest Rate [Member] | Less Than Six Months Remaining Maturity [Member]      
Derivative [Line Items]      
Cost Basis $ 16,260,000 $ 0  
Fair Value $ (4,694,000) $ 2,353,000  
Weighted Average Remaining Maturity 5 months 5 days 2 months 9 days  
Underlying Swap [Member] | Long [Member] | Variable Income Interest Rate [Member] | Less Than Six Months Remaining Maturity [Member]      
Derivative [Line Items]      
Weighted Average Remaining Maturity 3 years 9 months 4 years 9 months 10 days  
Notional $ 7,200,000,000 $ 4,500,000,000  
Underlying Swap [Member] | Long [Member] | Variable Income Interest Rate [Member] | Less Than Six Months Remaining Maturity [Member] | London Interbank Offered Rate (LIBOR) [Member]      
Derivative [Line Items]      
Weighted Average Fixed Interest Rate 2.27% 2.16%  
Underlying Swap [Member] | Long [Member] | Variable Income Interest Rate [Member] | Six Months or Longer Remaining Maturity [Member]      
Derivative [Line Items]      
Weighted Average Remaining Maturity   10 years  
Notional   $ 300,000,000  
Underlying Swap [Member] | Long [Member] | Variable Income Interest Rate [Member] | Six Months or Longer Remaining Maturity [Member] | London Interbank Offered Rate (LIBOR) [Member]      
Derivative [Line Items]      
Weighted Average Fixed Interest Rate   3.50%  
Underlying Swap [Member] | Long [Member] | Fixed Income Interest Rate [Member] | Less Than Six Months Remaining Maturity [Member]      
Derivative [Line Items]      
Weighted Average Remaining Maturity 10 years    
Notional $ 2,300,000,000    
Underlying Swap [Member] | Long [Member] | Fixed Income Interest Rate [Member] | Less Than Six Months Remaining Maturity [Member] | London Interbank Offered Rate (LIBOR) [Member]      
Derivative [Line Items]      
Weighted Average Fixed Interest Rate 2.10%    
Underlying Swap [Member] | Short [Member] | Variable Income Interest Rate [Member] | Less Than Six Months Remaining Maturity [Member]      
Derivative [Line Items]      
Weighted Average Remaining Maturity 10 years 10 years  
Notional $ 1,693,000,000 $ 500,000,000  
Underlying Swap [Member] | Short [Member] | Variable Income Interest Rate [Member] | Less Than Six Months Remaining Maturity [Member] | London Interbank Offered Rate (LIBOR) [Member]      
Derivative [Line Items]      
Weighted Average Fixed Interest Rate 2.70% 3.40%  
Underlying Swap [Member] | Short [Member] | Variable Income Interest Rate [Member] | Six Months or Longer Remaining Maturity [Member]      
Derivative [Line Items]      
Weighted Average Remaining Maturity   10 years  
Notional   $ 300,000,000  
Underlying Swap [Member] | Short [Member] | Variable Income Interest Rate [Member] | Six Months or Longer Remaining Maturity [Member] | London Interbank Offered Rate (LIBOR) [Member]      
Derivative [Line Items]      
Weighted Average Fixed Interest Rate   3.50%  
Underlying Swap [Member] | Short [Member] | Fixed Income Interest Rate [Member] | Less Than Six Months Remaining Maturity [Member]      
Derivative [Line Items]      
Weighted Average Remaining Maturity 5 years 7 months 9 days 4 years 11 months 8 days  
Notional $ 5,141,000,000 $ 3,775,000,000  
Underlying Swap [Member] | Short [Member] | Fixed Income Interest Rate [Member] | Less Than Six Months Remaining Maturity [Member] | London Interbank Offered Rate (LIBOR) [Member]      
Derivative [Line Items]      
Weighted Average Fixed Interest Rate 1.89% 1.19%