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, 2017
Jun. 30, 2017
Dec. 31, 2016
Sep. 30, 2016
Jun. 30, 2016
Dec. 31, 2015
Derivative [Line Items]            
Notional $ 24,113,321,000 $ 16,988,116,000 $ 18,802,500,000 $ 22,062,904,000 $ 26,141,370,000 $ 21,998,381,000
Total Return Swap [Member]            
Derivative [Line Items]            
Notional 65,895,000   90,593,000      
Fair Value 393,000   (17,000)      
Cost Basis 567,000   686,000      
Derivative, Unrealized Gains (Losses) (174,000)   (703,000)      
Total Return Swap [Member] | Maturity Date, 1/12/2043 [Member]            
Derivative [Line Items]            
Notional 25,262,150   45,083,136      
Fair Value 124,000   (5,000)      
Cost Basis 201,000   320,000      
Derivative, Unrealized Gains (Losses) (77,000)   (325,000)      
Total Return Swap [Member] | Maturity Date, 1/12/2044 [Member]            
Derivative [Line Items]            
Notional 40,633,058   45,509,532      
Fair Value 269,000   (12,000)      
Cost Basis 366,000   366,000      
Derivative, Unrealized Gains (Losses) $ (97,000)   $ (378,000)