Quarterly report pursuant to Section 13 or 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 ($)
 3 Months Ended 12 Months Ended
Mar. 31, 2018
Dec. 31, 2017
Mar. 31, 2017
Dec. 31, 2016
Derivative [Line Items]        
Notional $ (18,429,288,000) $ (31,226,878,000) $ (15,935,535,000) $ (18,802,500,000)
Interest Rate Swaption [Member] | Long [Member] | Variable Income Interest Rate [Member] | Less Than Six Months Remaining Maturity [Member]        
Derivative [Line Items]        
Cost Basis (8,461,000) (21,380,000)    
Fair Value $ 10,754,000 $ 17,736,000    
Weighted Average Remaining Maturity 3 months 20 days 4 months 1 day    
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)    
Fair Value   $ 5,619,000    
Weighted Average Remaining Maturity   4 months 20 days    
Interest Rate Swaption [Member] | Short [Member] | Variable Income Interest Rate [Member] | Six Months or Longer Remaining Maturity [Member]        
Derivative [Line Items]        
Cost Basis $ (6,808,000)      
Fair Value $ (6,783,000)      
Weighted Average Remaining Maturity 11 months 6 days      
Interest Rate Swaption [Member] | Short [Member] | Fixed Income Interest Rate [Member] | Less Than Six Months Remaining Maturity [Member]        
Derivative [Line Items]        
Cost Basis $ (23,580,000) $ (16,260,000)    
Fair Value $ (9,301,000) $ 4,694,000    
Weighted Average Remaining Maturity 2 months 15 days 5 months 5 days    
Interest Rate Swaption [Member] | Short [Member] | Fixed Income Interest Rate [Member] | Six Months or Longer Remaining Maturity [Member]        
Derivative [Line Items]        
Cost Basis $ (6,962,000)      
Fair Value $ (6,501,000)      
Weighted Average Remaining Maturity 11 months 6 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 5 years 3 years 9 months    
Notional $ (4,095,000,000) $ (7,200,000,000)    
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 2.98% 2.27%    
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)    
Weighted Average Fixed Interest Rate   2.095%    
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    
Notional   $ (1,693,000,000)    
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%    
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 $ (280,000,000)      
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 2.99%      
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 5 years 7 months 9 days    
Notional $ (9,710,000,000) $ (5,141,000,000)    
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 2.32% 1.89%    
Underlying Swap [Member] | Short [Member] | Fixed Income Interest Rate [Member] | Six Months or Longer Remaining Maturity [Member]        
Derivative [Line Items]        
Weighted Average Remaining Maturity 10 years      
Notional $ (280,000,000)      
Underlying Swap [Member] | Short [Member] | Fixed 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 2.99%