Securities-Aggregate Fair Value of Investments with Continuous Unrealized Loss Position (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013		Dec. 31, 2012
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair value	$ 11,385		$ 5,758
Less than 12 months Unrealized losses	88		63
12 months or more Fair value	2,808		2,798
12 months or more Unrealized losses	270		300
Total Fair value	14,193		8,556
Total Unrealized losses	358		363
Available-for-sale
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair value	10,546		5,073
Less than 12 months Unrealized losses	86		62
12 months or more Fair value	2,338		2,296
12 months or more Unrealized losses	218		239
Total Fair value	12,884		7,369
Total Unrealized losses	304		301
Available-for-sale | U.S. Treasury
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair value	2,190		956
Less than 12 months Unrealized losses	36		3
12 months or more Fair value	0		0
12 months or more Unrealized losses	0		0
Total Fair value	2,190		956
Total Unrealized losses	36		3
Available-for-sale | State and political subdivisions
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair value	612		1,139
Less than 12 months Unrealized losses	5		7
12 months or more Fair value	306		173
12 months or more Unrealized losses	23		22
Total Fair value	918		1,312
Total Unrealized losses	28		29
Available-for-sale | Agency RMBS
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair value	6,061		1,336
Less than 12 months Unrealized losses	30		8
12 months or more Fair value	96		96
12 months or more Unrealized losses	0		0
Total Fair value	6,157		1,432
Total Unrealized losses	30		8
Available-for-sale | Alt-A RMBS
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair value	13		31
Less than 12 months Unrealized losses	3		13
12 months or more Fair value	37		39
12 months or more Unrealized losses	10		3
Total Fair value	50		70
Total Unrealized losses	13		16
Available-for-sale | Prime RMBS
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair value	66		110
Less than 12 months Unrealized losses	1		2
12 months or more Fair value	219		253
12 months or more Unrealized losses	6		7
Total Fair value	285		363
Total Unrealized losses	7		9
Available-for-sale | Subprime RMBS
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair value	0		13
Less than 12 months Unrealized losses	0		3
12 months or more Fair value	413		397
12 months or more Unrealized losses	41		59
Total Fair value	413		410
Total Unrealized losses	41		62
Available-for-sale | Other RMBS
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair value	144		64
Less than 12 months Unrealized losses	1		19
12 months or more Fair value	635		670
12 months or more Unrealized losses	93		90
Total Fair value	779		734
Total Unrealized losses	94		109
Available-for-sale | Commercial MBS
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair value	285		131
Less than 12 months Unrealized losses	2		1
12 months or more Fair value	301		310
12 months or more Unrealized losses	37		44
Total Fair value	586		441
Total Unrealized losses	39		45
Available-for-sale | Asset-backed CLOs
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair value	141		314
Less than 12 months Unrealized losses	1		1
12 months or more Fair value	297		321
12 months or more Unrealized losses	4		9
Total Fair value	438		635
Total Unrealized losses	5		10
Available-for-sale | Other asset-backed securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair value	832		779
Less than 12 months Unrealized losses	2		2
12 months or more Fair value	9		7
12 months or more Unrealized losses	1		1
Total Fair value	841		786
Total Unrealized losses	3		3
Available-for-sale | Corporate bonds
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair value	198		178
Less than 12 months Unrealized losses	5		3
12 months or more Fair value	0		0
12 months or more Unrealized losses	0		0
Total Fair value	198		178
Total Unrealized losses	5		3
Available-for-sale | Alt-A RMBS
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair value	4	[1]	22	[1]
Less than 12 months Unrealized losses	0	[1]	0	[1]
12 months or more Fair value	25	[1]	30	[1]
12 months or more Unrealized losses	3	[1]	4	[1]
Total Fair value	29	[1]	52	[1]
Total Unrealized losses	3	[1]	4	[1]
Held-to-maturity
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair value	839		685
Less than 12 months Unrealized losses	2		1
12 months or more Fair value	470		502
12 months or more Unrealized losses	52		61
Total Fair value	1,309		1,187
Total Unrealized losses	54		62
Held-to-maturity | U.S. Treasury
Investments, Unrealized Loss Position [Line Items]
Total Unrealized losses	0		0
Held-to-maturity | State and political subdivisions
Investments, Unrealized Loss Position [Line Items]
Total Unrealized losses	0		0
Held-to-maturity | Agency RMBS
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair value	839		234
Less than 12 months Unrealized losses	2		1
12 months or more Fair value	0		0
12 months or more Unrealized losses	0		0
Total Fair value	839		234
Total Unrealized losses	2		1
Held-to-maturity | Alt-A RMBS
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair value	0		38
Less than 12 months Unrealized losses	0		0
12 months or more Fair value	20		24
12 months or more Unrealized losses	6		6
Total Fair value	20		62
Total Unrealized losses	6		6
Held-to-maturity | Prime RMBS
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair value			0
Less than 12 months Unrealized losses			0
12 months or more Fair value			56
12 months or more Unrealized losses			1
Total Fair value			56
Total Unrealized losses	0		1
Held-to-maturity | Subprime RMBS
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair value			0
Less than 12 months Unrealized losses			0
12 months or more Fair value			24
12 months or more Unrealized losses			1
Total Fair value			24
Total Unrealized losses	0		1
Held-to-maturity | Other RMBS
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair value	0		413
Less than 12 months Unrealized losses	0		0
12 months or more Fair value	430		373
12 months or more Unrealized losses	45		52
Total Fair value	430		786
Total Unrealized losses	45		52
Held-to-maturity | Commercial MBS
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair value	0		0
Less than 12 months Unrealized losses	0		0
12 months or more Fair value	20		25
12 months or more Unrealized losses	1		1
Total Fair value	20		25
Total Unrealized losses	$ 1		$ 1

[1] Previously included in the Grantor Trust. The Grantor Trust was dissolved in 2011.