Annual report pursuant to Section 13 and 15(d)

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

v3.8.0.1
Derivative Instruments and Hedging Activities Schedule of Total Return Swaps (Details) - USD ($)
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Derivative [Line Items]      
Notional $ 31,226,878,000 $ 18,802,500,000 $ 21,998,381,000
Total Return Swap [Member]      
Derivative [Line Items]      
Notional 63,507,000 90,593,000  
Fair Value (106,000) (17,000)  
Cost Basis 567,000 686,000  
Derivative, Unrealized Gains (Losses) (673,000) (703,000)  
Total Return Swap [Member] | Maturity Date, 1/12/2043 [Member]      
Derivative [Line Items]      
Notional 24,362,621 45,083,136  
Fair Value (24,000) (5,000)  
Cost Basis 201,000 320,000  
Derivative, Unrealized Gains (Losses) (225,000) (325,000)  
Total Return Swap [Member] | Maturity Date, 1/12/2044 [Member]      
Derivative [Line Items]      
Notional 39,144,706 45,509,532  
Fair Value (82,000) (12,000)  
Cost Basis 366,000 366,000  
Derivative, Unrealized Gains (Losses) $ (448,000) $ (378,000)