Schedule of Agency Securities Portfolio Backed by U.S. Government Agency or U.S. Government Sponsored Entity (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012
Schedule Of Available For Sale Securities [Line Items]
Agency Securities Amortized Cost	$ 17,533,042	$ 23,419,908
Gross Unrealized Loss	(120,019)	(121)
Gross Unrealized Gain	229,509	499,464
Carrying value/estimated fair value	17,642,532	23,919,251
Federal National Mortgage Association Certificates and Obligations (FNMA)
Schedule Of Available For Sale Securities [Line Items]
Agency Securities Amortized Cost	10,427,055	14,824,558
Gross Unrealized Loss	(46,669)	(100)
Gross Unrealized Gain	171,680	339,076
Carrying value/estimated fair value	10,552,066	15,163,534
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises
Schedule Of Available For Sale Securities [Line Items]
Agency Securities Amortized Cost	7,009,751	8,595,350
Gross Unrealized Loss	(72,352)	(21)
Gross Unrealized Gain	57,829	160,388
Carrying value/estimated fair value	6,995,228	8,755,717
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	9,620,743	14,081,259
Gross Unrealized Loss	(44,871)	(100)
Gross Unrealized Gain	167,848	329,780
Carrying value/estimated fair value	9,743,720	14,410,939
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	806,312	743,299
Gross Unrealized Loss	(1,798)
Gross Unrealized Gain	3,832	9,296
Carrying value/estimated fair value	808,346	752,595
Freddie Mac Certificates, ARMs | Mortgage-backed Securities, Issued by US Government Sponsored Enterprises
Schedule Of Available For Sale Securities [Line Items]
Agency Securities Amortized Cost	6,671,013	7,850,630
Gross Unrealized Loss	(70,752)	(21)
Gross Unrealized Gain	57,808	149,114
Carrying value/estimated fair value	6,658,069	7,999,723
Freddie Mac Certificates, Fixed Rate | Mortgage-backed Securities, Issued by US Government Sponsored Enterprises
Schedule Of Available For Sale Securities [Line Items]
Agency Securities Amortized Cost	338,738	744,720
Gross Unrealized Loss	(1,600)
Gross Unrealized Gain	21	11,274
Carrying value/estimated fair value	337,159	755,994
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 Rates | 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