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 ($)
Sep. 30, 2018
Jun. 30, 2018
Dec. 31, 2017
Sep. 30, 2017
Jun. 30, 2017
Dec. 31, 2016
Derivative [Line Items]            
Notional $ 41,330,283,000 $ 28,618,861,000 $ 31,226,878,000 $ 24,113,321,000 $ 16,988,116,000 $ 18,802,500,000
Total Return Swap [Member]            
Derivative [Line Items]            
Notional 49,691,000   63,507,000      
Fair Value 2,000   (106,000)      
Cost Basis (59,000)   567,000      
Derivative, Unrealized Gains (Losses) 61,000   (673,000)      
Total Return Swap [Member] | Maturity Date, 1/12/2043 [Member]            
Derivative [Line Items]            
Notional 22,010,000   24,362,000      
Fair Value (1,000)   (24,000)      
Cost Basis (30,000)   201,000      
Derivative, Unrealized Gains (Losses) 29,000   (225,000)      
Total Return Swap [Member] | Maturity Date, 1/12/2044 [Member]            
Derivative [Line Items]            
Notional 27,681,000   39,145,000      
Fair Value 3,000   (82,000)      
Cost Basis (29,000)   366,000      
Derivative, Unrealized Gains (Losses) $ 32,000   $ (448,000)