Schedule of Agency Securities and Non Agency Securities Portfolio MBS not Issued or Guaranteed by U.S. Government Agency or U.S. Government Sponsored Entity (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Schedule Of Available For Sale Securities [Line Items]
Agency Securities Amortized Cost	$ 16,697,285	$ 17,533,042
Gross Unrealized Loss	(52,335)	(120,019)
Gross Unrealized Gain	245,474	229,509
Carrying value/estimated fair value	16,890,424	17,642,532
Federal National Mortgage Association Certificates and Obligations (FNMA)
Schedule Of Available For Sale Securities [Line Items]
Agency Securities Amortized Cost	9,786,795	10,427,055
Gross Unrealized Loss	(20,619)	(46,669)
Gross Unrealized Gain	178,079	171,680
Carrying value/estimated fair value	9,944,255	10,552,066
Freddie Mac Certificates
Schedule Of Available For Sale Securities [Line Items]
Agency Securities Amortized Cost	6,834,290	7,009,751
Gross Unrealized Loss	(31,266)	(72,352)
Gross Unrealized Gain	67,395	57,829
Carrying value/estimated fair value	6,870,419	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)
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	9,217,059	9,620,743
Gross Unrealized Loss	(20,004)	(44,871)
Gross Unrealized Gain	175,699	167,848
Carrying value/estimated fair value	9,372,754	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	569,736	806,312
Gross Unrealized Loss	(615)	(1,798)
Gross Unrealized Gain	2,380	3,832
Carrying value/estimated fair value	571,501	808,346
Freddie Mac Certificates, ARMs | Freddie Mac Certificates
Schedule Of Available For Sale Securities [Line Items]
Agency Securities Amortized Cost	6,667,896	6,671,013
Gross Unrealized Loss	(31,236)	(70,752)
Gross Unrealized Gain	66,537	57,808
Carrying value/estimated fair value	6,703,197	6,658,069
Freddie Mac Certificates, Fixed Rate | Freddie Mac Certificates
Schedule Of Available For Sale Securities [Line Items]
Agency Securities Amortized Cost	166,394	338,738
Gross Unrealized Loss	(30)	(1,600)
Gross Unrealized Gain	858	21
Carrying value/estimated fair value	167,222	337,159
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)
Carrying value/estimated fair value		95,238
Agency Securities
Schedule Of Available For Sale Securities [Line Items]
Agency Securities Amortized Cost	16,621,085
Gross Unrealized Loss	(51,885)
Gross Unrealized Gain	245,474
Carrying value/estimated fair value	16,814,674
Non-Agency ARMs
Schedule Of Available For Sale Securities [Line Items]
Agency Securities Amortized Cost	76,200
Gross Unrealized Loss	(450)
Gross Unrealized Gain	0
Carrying value/estimated fair value	$ 75,750