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 ($)
Mar. 31, 2018
Dec. 31, 2017
Mar. 31, 2017
Dec. 31, 2016
Derivative [Line Items]        
Notional $ 18,429,288,000 $ 31,226,878,000 $ 15,935,535,000 $ 18,802,500,000
Total Return Swap [Member]        
Derivative [Line Items]        
Notional 61,521,000 63,507,000    
Fair Value (176,000) (106,000)    
Cost Basis 567,000 567,000    
Derivative, Unrealized Gains (Losses) (743,000) (673,000)    
Total Return Swap [Member] | Maturity Date, 1/12/2043 [Member]        
Derivative [Line Items]        
Notional 23,607,837 24,362,621    
Fair Value (63,000) (24,000)    
Cost Basis 201,000 201,000    
Derivative, Unrealized Gains (Losses) (264,000) (225,000)    
Total Return Swap [Member] | Maturity Date, 1/12/2044 [Member]        
Derivative [Line Items]        
Notional 37,913,368 39,144,706    
Fair Value (113,000) (82,000)    
Cost Basis 366,000 366,000    
Derivative, Unrealized Gains (Losses) $ (479,000) $ (448,000)