Quarterly report pursuant to Section 13 or 15(d)

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

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Derivative Instruments and Hedging Activities Schedule of Interest Rate Swaptions (Details) - USD ($)
 9 Months Ended 12 Months Ended
Sep. 30, 2018
Dec. 31, 2017
Jun. 30, 2018
Sep. 30, 2017
Jun. 30, 2017
Dec. 31, 2016
Derivative [Line Items]            
Notional $ (41,330,283,000) $ (31,226,878,000) $ (28,618,861,000) $ (24,113,321,000) $ (16,988,116,000) $ (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 (9,400,000) (21,380,000)        
Fair Value $ 30,608,000 $ 17,736,000        
Weighted Average Remaining Maturity 3 months 19 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] | Fixed Income Interest Rate [Member] | Less Than Six Months Remaining Maturity [Member]            
Derivative [Line Items]            
Cost Basis $ (9,730,000) $ (16,260,000)        
Fair Value $ (5,696,000) $ 4,694,000        
Weighted Average Remaining Maturity 3 months 27 days 5 months 5 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 7 years 7 months 6 days 3 years 9 months 18 days        
Notional $ (5,225,000,000) $ (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 3.20% 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   2.10%        
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        
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] | Fixed Income Interest Rate [Member] | Less Than Six Months Remaining Maturity [Member]            
Derivative [Line Items]            
Weighted Average Remaining Maturity 7 years 8 months 12 days 5 years 7 months 6 days        
Notional $ (5,061,000,000) $ (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 2.70% 1.89%