Derivative Instruments and Hedging Activities Schedule of Total Return Swaps (Details) - USD ($) $ in Thousands	Sep. 30, 2015	Jun. 30, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Dec. 31, 2013
Derivative [Line Items]
Derivative, Notional Amount	$ (28,317,580)	$ (27,401,335)	$ (48,115,523)	$ (57,015,981)	$ (43,379,217)	$ (37,366,610)
Total Return Swap [Member]
Derivative [Line Items]
Derivative, Notional Amount	(887,964)		(598,459)
Derivative, Fair Value, Net	(4,992)		1,387
Derivative, Cost	(1,763)		(1,732)
Derivative, Unrealized Gains (Losses)	(6,755)		(345)
Maturity Date, 1/12/2043 [Member] | Total Return Swap [Member]
Derivative [Line Items]
Derivative, Notional Amount	(378,890)		(411,281)
Derivative, Fair Value, Net	(1,808)		763
Derivative, Cost	(866)		(1,457)
Derivative, Unrealized Gains (Losses)	(2,674)		(694)
Maturity Date, 1/12/2044 [Member] | Total Return Swap [Member]
Derivative [Line Items]
Derivative, Notional Amount	(332,924)		(187,178)
Derivative, Fair Value, Net	(1,589)		624
Derivative, Cost	(1,679)		(275)
Derivative, Unrealized Gains (Losses)	(3,268)		349
Maturity Date, 1/12/2045 [Member] | Total Return Swap [Member]
Derivative [Line Items]
Derivative, Notional Amount	(176,150)
Derivative, Fair Value, Net	(1,595)
Derivative, Cost	782
Derivative, Unrealized Gains (Losses)	(813)
Derivative Financial Instruments, Assets [Member]
Derivative [Line Items]
Derivative, Notional Amount	(18,899,975)		(36,315,523)
Derivative, Fair Value, Net	$ 296,731		$ 380,791