Schedule Of Mortgage-Backed Securities Portfolio Backed By U.S. Government Agency Or U.S. Government Sponsored Entity (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Schedule of Available-for-sale Securities [Line Items]
MBS Amortized Cost	$ 21,910,788	$ 17,356,772
Gross Unrealized Loss		(138)
Gross Unrealized Gain	456,333	385,239
Carrying value/estimated fair value	22,367,121	17,741,873
Fannie Mae Certificates, ARMS
Schedule of Available-for-sale Securities [Line Items]
MBS Amortized Cost	13,443,060	11,446,397
Gross Unrealized Gain	309,182	278,559
Carrying value/estimated fair value	13,752,242	11,724,956
Fannie Mae Certificates, Fixed Rate
Schedule of Available-for-sale Securities [Line Items]
MBS Amortized Cost	1,028,329	493,648
Gross Unrealized Loss		(132)
Gross Unrealized Gain	13,067	1,076
Carrying value/estimated fair value	1,041,396	494,592
Federal National Mortgage Association Certificates and Obligations (FNMA)
Schedule of Available-for-sale Securities [Line Items]
MBS Amortized Cost	7,439,399	11,940,045
Gross Unrealized Loss		(132)
Gross Unrealized Gain	134,084	279,635
Carrying value/estimated fair value	7,573,483	12,219,548
Freddie Mac Certificates, ARMS
Schedule of Available-for-sale Securities [Line Items]
MBS Amortized Cost	6,850,805	4,925,438
Gross Unrealized Gain	126,621	103,540
Carrying value/estimated fair value	6,977,426	5,028,978
Freddie Mac Certificates, Fixed Rate
Schedule of Available-for-sale Securities [Line Items]
MBS Amortized Cost	588,594	491,289
Gross Unrealized Loss		(6)
Gross Unrealized Gain	7,463	2,064
Carrying value/estimated fair value	596,057	493,347
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises
Schedule of Available-for-sale Securities [Line Items]
MBS Amortized Cost	14,471,389	5,416,727
Gross Unrealized Loss		(6)
Gross Unrealized Gain	322,249	105,604
Carrying value/estimated fair value	$ 14,793,638	$ 5,522,325