Schedule of Agency Securities Portfolio Backed by U.S. Government Agency or U.S. Government Sponsored Entity (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Schedule of Available-for-sale Securities [Line Items]
Agency Securities Amortized Cost	$ 23,419,908	$ 17,356,772
Gross Unrealized Loss	(121)	(138)
Gross Unrealized Gain	499,464	385,239
Carrying value/estimated fair value	23,919,251	17,741,873
Federal National Mortgage Association Certificates and Obligations (FNMA)
Schedule of Available-for-sale Securities [Line Items]
Agency Securities Amortized Cost	14,824,558	11,940,045
Gross Unrealized Loss	(100)	(132)
Gross Unrealized Gain	339,076	279,635
Carrying value/estimated fair value	15,163,534	12,219,548
Federal National Mortgage Association Certificates and Obligations (FNMA) | Fannie Mae Certificates, ARMs
Schedule of Available-for-sale Securities [Line Items]
Agency Securities Amortized Cost	14,081,259	11,446,397
Gross Unrealized Loss	(100)
Gross Unrealized Gain	329,780	278,559
Carrying value/estimated fair value	14,410,939	11,724,956
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	743,299	493,648
Gross Unrealized Loss		(132)
Gross Unrealized Gain	9,296	1,076
Carrying value/estimated fair value	752,595	494,592
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises
Schedule of Available-for-sale Securities [Line Items]
Agency Securities Amortized Cost	8,595,350	5,416,727
Gross Unrealized Loss	(21)	(6)
Gross Unrealized Gain	160,388	105,604
Carrying value/estimated fair value	8,755,717	5,522,325
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	7,850,630	4,925,438
Gross Unrealized Loss	(21)
Gross Unrealized Gain	149,114	103,540
Carrying value/estimated fair value	7,999,723	5,028,978
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	744,720	491,289
Gross Unrealized Loss		(6)
Gross Unrealized Gain	11,274	2,064
Carrying value/estimated fair value	$ 755,994	$ 493,347