Offsetting Assets and Liabilities Offsettting Liabilities (Details) - USD ($) $ in Thousands		Sep. 30, 2015	Dec. 31, 2014
Offsetting Liabilities [Line Items]
Derivative liabilities, fair value, gross liability		$ 63,708	$ 35,898
Derivative liabilities, fair value, gross amount offset in balance sheet		0	0
Derivative liabilities, fair value, amount not offset against collateral		63,708	35,898
Derivative, Collateral, Right to Reclaim Securities	[1]	(13,851)	(2,494)
Derivative, Collateral, Right to Reclaim Cash	[1]	(49,581)	(32,994)
Derivative liabilities, fair value, amount offset against collateral		276	410
Repurchase agreements, amount not offset against collateral		3,055,069	3,013,110
Total gross liabilities		3,373,777	3,049,008
Total gross liabilities, gross amount offset in balance sheet		0	0
Total liabilities, amount not offset against collateral		3,373,777	3,049,008
Total liabilities, Collateral, Right to Reclaim Securities	[1]	(3,323,920)	(3,015,604)
Total liabilities, Collateral, Right to Reclaim Cash	[1]	(49,581)	(32,994)
Total liabilities, amount offset against collateral		276	410
Repurchase agreements [Member]
Offsetting Liabilities [Line Items]
Repurchase agreements, gross liability		3,055,069	3,013,110
Repurchase agreements, gross amount offset in balance sheet		0	0
Repurchase agreements, amount not offset against collateral		3,055,069	3,013,110
Repurchase agreements, Collateral, Right to Reclaim Securities	[1]	(3,055,069)	(3,013,110)
Repurchase agreements, Collateral, Right to Reclaim Cash	[1]	0	0
Repurchase agreements, amount offset against collateral		0	$ 0
FHLB Advances [Member]
Offsetting Liabilities [Line Items]
Repurchase agreements, gross liability		255,000
Repurchase agreements, gross amount offset in balance sheet		0
Repurchase agreements, amount not offset against collateral		255,000
Repurchase agreements, Collateral, Right to Reclaim Securities		(255,000)
Repurchase agreements, Collateral, Right to Reclaim Cash		0
Repurchase agreements, amount offset against collateral		$ 0

[1] Amount disclosed for collateral received by or posted to the same counterparty include cash and the fair value of MBS up to and not exceeding the net amount of the asset or liability presented in the balance sheet. The fair value of the actual collateral received by or posted to the same counterparty may exceed the amounts presented.