Annual report pursuant to Section 13 and 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 ($)
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Derivative [Line Items]      
Notional $ 49,205,497,000 $ 36,528,169,000 $ 31,226,878,000
Markit IOS Total Return Swap [Member]      
Derivative [Line Items]      
Notional 41,890,000 48,265,000  
Fair Value (29,000) (383,000)  
Cost Basis (59,000) (59,000)  
Derivative, Unrealized Gains (Losses) 30,000 (324,000)  
Markit IOS Total Return Swap [Member] | Maturity Date, 1/12/2043 [Member]      
Derivative [Line Items]      
Notional 18,625,000 21,395,000  
Fair Value 5,000 (153,000)  
Cost Basis (30,000) (30,000)  
Derivative, Unrealized Gains (Losses) 35,000 (123,000)  
Markit IOS Total Return Swap [Member] | Maturity Date, 1/12/2044 [Member]      
Derivative [Line Items]      
Notional 23,265,000 26,870,000  
Fair Value (34,000) (230,000)  
Cost Basis (29,000) (29,000)  
Derivative, Unrealized Gains (Losses) $ (5,000) $ (201,000)