Schedule of Agency Securities Portfolio Backed by U.S. Government Agency or U.S. Government Sponsored Entity (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items]
Agency Securities Amortized Cost	$ 24,681,406	$ 23,419,908
Gross Unrealized Loss	(1,568)	(121)
Gross Unrealized Gain	482,892	499,464
Carrying value/estimated fair value	25,162,730	23,919,251
Federal National Mortgage Association Certificates and Obligations (FNMA)
Schedule of Available-for-sale Securities [Line Items]
Agency Securities Amortized Cost	15,623,030	14,824,558
Gross Unrealized Loss	(1,540)	(100)
Gross Unrealized Gain	330,575	339,076
Carrying value/estimated fair value	15,952,065	15,163,534
Federal National Mortgage Association Certificates and Obligations (FNMA) | Fannie Mae Certificates, ARMs
Schedule of Available-for-sale Securities [Line Items]
Agency Securities Amortized Cost	13,895,410	14,081,259
Gross Unrealized Loss	(83)	(100)
Gross Unrealized Gain	325,422	329,780
Carrying value/estimated fair value	14,220,749	14,410,939
Federal National Mortgage Association Certificates and Obligations (FNMA) | Fannie Mae Certificates, Fixed Rate
Schedule of Available-for-sale Securities [Line Items]
Agency Securities Amortized Cost	1,727,620	743,299
Gross Unrealized Loss	(1,457)
Gross Unrealized Gain	5,153	9,296
Carrying value/estimated fair value	1,731,316	752,595
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises
Schedule of Available-for-sale Securities [Line Items]
Agency Securities Amortized Cost	9,058,376	8,595,350
Gross Unrealized Loss	(28)	(21)
Gross Unrealized Gain	152,317	160,388
Carrying value/estimated fair value	9,210,665	8,755,717
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises | Freddie Mac Certificates, ARMs
Schedule of Available-for-sale Securities [Line Items]
Agency Securities Amortized Cost	8,186,654	7,850,630
Gross Unrealized Loss	(20)	(21)
Gross Unrealized Gain	146,196	149,114
Carrying value/estimated fair value	8,332,830	7,999,723
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises | Freddie Mac Certificates, Fixed Rate
Schedule of Available-for-sale Securities [Line Items]
Agency Securities Amortized Cost	871,722	744,720
Gross Unrealized Loss	(8)
Gross Unrealized Gain	6,121	11,274
Carrying value/estimated fair value	$ 877,835	$ 755,994