Quarterly report pursuant to Section 13 or 15(d)

Derivative Instruments and Hedging Activities Schedule of Total Return Swaps (Details)

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Derivative Instruments and Hedging Activities Schedule of Total Return Swaps (Details) - USD ($)
Jun. 30, 2018
Mar. 31, 2018
Dec. 31, 2017
Jun. 30, 2017
Mar. 31, 2017
Dec. 31, 2016
Derivative [Line Items]            
Notional $ 28,618,861,000 $ 18,429,288,000 $ 31,226,878,000 $ 16,988,116,000 $ 15,935,535,000 $ 18,802,500,000
Total Return Swap [Member]            
Derivative [Line Items]            
Notional 51,541,000   63,507,000      
Fair Value (295,000)   (106,000)      
Cost Basis 489,000   567,000      
Derivative, Unrealized Gains (Losses) (784,000)   (673,000)      
Total Return Swap [Member] | Maturity Date, 1/12/2043 [Member]            
Derivative [Line Items]            
Notional 22,808,263   24,362,621      
Fair Value (117,000)   (24,000)      
Cost Basis 201,000   201,000      
Derivative, Unrealized Gains (Losses) (318,000)   (225,000)      
Total Return Swap [Member] | Maturity Date, 1/12/2044 [Member]            
Derivative [Line Items]            
Notional 28,732,523   39,144,706      
Fair Value (178,000)   (82,000)      
Cost Basis 288,000   366,000      
Derivative, Unrealized Gains (Losses) $ (466,000)   $ (448,000)