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)