Derivative Instruments and Hedging Activities Schedule of Total Return Swaps (Details) - USD ($) $ in Thousands	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014
Derivative [Line Items]
Derivative, Notional Amount	$ (26,141,370)	$ (26,390,596)	$ (21,998,381)	$ (27,401,335)	$ (33,709,043)	$ (48,115,523)
Total Return Swap [Member]
Derivative [Line Items]
Derivative, Notional Amount	(588,037)	$ (868,145)	(889,418)	$ (988,409)	$ (877,529)	$ (598,459)
Derivative, Fair Value, Net	(7,419)		1,645
Derivative, Cost	3,539		1,383
Derivative, Unrealized Gains (Losses)	(10,958)		262
Maturity Date, 1/12/2043 [Member] | Total Return Swap [Member]
Derivative [Line Items]
Derivative, Notional Amount	(275,900)		(369,639)
Derivative, Fair Value, Net	(3,455)		456
Derivative, Cost	1,818		866
Derivative, Unrealized Gains (Losses)	(5,273)		(410)
Maturity Date, 1/12/2044 [Member] | Total Return Swap [Member]
Derivative [Line Items]
Derivative, Notional Amount	(257,720)		(325,003)
Derivative, Fair Value, Net	(3,187)		350
Derivative, Cost	2,346		1,679
Derivative, Unrealized Gains (Losses)	(5,533)		(1,329)
Maturity Date, 1/12/2045 [Member] | Total Return Swap [Member]
Derivative [Line Items]
Derivative, Notional Amount	(54,417)		(194,776)
Derivative, Fair Value, Net	(777)		839
Derivative, Cost	(625)		1,162
Derivative, Unrealized Gains (Losses)	(152)		2,001
Derivative Financial Instruments, Assets [Member]
Derivative [Line Items]
Derivative, Notional Amount	(16,940,106)		(21,735,997)
Derivative, Fair Value, Net	223,985		271,509
Derivative Financial Instruments, Assets [Member] | Total Return Swap [Member]
Derivative [Line Items]
Derivative, Notional Amount	0		(889,418)
Derivative, Fair Value, Net	$ 0		$ 1,645