Schedule of Agency Securities Portfolio Backed by U.S. Government Agency or U.S. Government Sponsored Entity (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Schedule Of Available For Sale Securities [Line Items]
Agency Securities Amortized Cost	$ 16,994,200	$ 17,533,042
Gross Unrealized Loss	(91,717)	(120,019)
Gross Unrealized Gain	235,473	229,509
Carrying value/estimated fair value	17,137,956	17,642,532
Federal National Mortgage Association Certificates and Obligations (FNMA)
Schedule Of Available For Sale Securities [Line Items]
Agency Securities Amortized Cost	9,856,932	10,427,055
Gross Unrealized Loss	(36,787)	(46,669)
Gross Unrealized Gain	172,301	171,680
Carrying value/estimated fair value	9,992,446	10,552,066
Freddie Mac Certificates
Schedule Of Available For Sale Securities [Line Items]
Agency Securities Amortized Cost	7,137,268	7,009,751
Gross Unrealized Loss	(54,930)	(72,352)
Gross Unrealized Gain	63,172	57,829
Carrying value/estimated fair value	7,145,510	6,995,228
Government National Mortgage Association Certificates and Obligations (GNMA)
Schedule Of Available For Sale Securities [Line Items]
Agency Securities Amortized Cost		96,236
Gross Unrealized Loss		(998)
Gross Unrealized Gain
Carrying value/estimated fair value		95,238
Fannie Mae Certificates, ARMs | Federal National Mortgage Association Certificates and Obligations (FNMA)
Schedule Of Available For Sale Securities [Line Items]
Agency Securities Amortized Cost	9,553,321	9,620,743
Gross Unrealized Loss	(36,442)	(44,871)
Gross Unrealized Gain	171,484	167,848
Carrying value/estimated fair value	9,688,363	9,743,720
Fannie Mae Certificates, Fixed Rate | Federal National Mortgage Association Certificates and Obligations (FNMA)
Schedule Of Available For Sale Securities [Line Items]
Agency Securities Amortized Cost	303,611	806,312
Gross Unrealized Loss	(345)	(1,798)
Gross Unrealized Gain	817	3,832
Carrying value/estimated fair value	304,083	808,346
Freddie Mac Certificates, ARMs | Freddie Mac Certificates
Schedule Of Available For Sale Securities [Line Items]
Agency Securities Amortized Cost	6,964,182	6,671,013
Gross Unrealized Loss	(54,734)	(70,752)
Gross Unrealized Gain	63,071	57,808
Carrying value/estimated fair value	6,972,519	6,658,069
Freddie Mac Certificates, Fixed Rate | Freddie Mac Certificates
Schedule Of Available For Sale Securities [Line Items]
Agency Securities Amortized Cost	173,086	338,738
Gross Unrealized Loss	(196)	(1,600)
Gross Unrealized Gain	101	21
Carrying value/estimated fair value	172,991	337,159
Ginnie Mae Certificates, Arms | Government National Mortgage Association Certificates and Obligations (GNMA)
Schedule Of Available For Sale Securities [Line Items]
Agency Securities Amortized Cost
Gross Unrealized Loss
Gross Unrealized Gain
Carrying value/estimated fair value
Ginnie Mae Certificates, Fixed Rate | Government National Mortgage Association Certificates and Obligations (GNMA)
Schedule Of Available For Sale Securities [Line Items]
Agency Securities Amortized Cost		96,236
Gross Unrealized Loss		(998)
Gross Unrealized Gain
Carrying value/estimated fair value		$ 95,238