Schedule of Agency Securities Portfolio Backed by U.S. Government Agency or U.S. Government Sponsored Entity (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Schedule of Available-for-sale Securities [Line Items]
Agency Securities Amortized Cost	$ 25,696,118	$ 17,356,772
Gross Unrealized Loss		(138)
Gross Unrealized Gain	679,019	385,239
Carrying value/estimated fair value	26,375,137	17,741,873
Federal National Mortgage Association Certificates and Obligations (FNMA)
Schedule of Available-for-sale Securities [Line Items]
Agency Securities Amortized Cost	16,999,233	11,940,045
Gross Unrealized Loss		(132)
Gross Unrealized Gain	467,046	279,635
Carrying value/estimated fair value	17,466,279	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	14,098,876	11,446,397
Gross Unrealized Gain	410,435	278,559
Carrying value/estimated fair value	14,509,311	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	2,900,357	493,648
Gross Unrealized Loss		(132)
Gross Unrealized Gain	56,611	1,076
Carrying value/estimated fair value	2,956,968	494,592
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises
Schedule of Available-for-sale Securities [Line Items]
Agency Securities Amortized Cost	8,696,885	5,416,727
Gross Unrealized Loss		(6)
Gross Unrealized Gain	211,973	105,604
Carrying value/estimated fair value	8,908,858	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	7,260,695	4,925,438
Gross Unrealized Gain	185,794	103,540
Carrying value/estimated fair value	7,446,489	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	1,436,190	491,289
Gross Unrealized Loss		(6)
Gross Unrealized Gain	26,179	2,064
Carrying value/estimated fair value	$ 1,462,369	$ 493,347