Securities with Unrealized Losses (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013		Dec. 31, 2012
Temporarily Impaired Securities
Investments, Unrealized Loss Position [Line Items]
Less than twelve months, Fair Value	$ 4,894		$ 1,511
Less than twelve months, Unrealized Losses	45		6
Twelve months or longer, Fair Value	40		37
Twelve months or longer, Unrealized Losses	3		7
Total, Fair Value	4,934		1,548
Total, Unrealized Losses	48		13
Temporarily Impaired Securities | US Government Agencies Debt Securities
Investments, Unrealized Loss Position [Line Items]
Less than twelve months, Fair Value	421		298
Less than twelve months, Unrealized Losses	5		3
Twelve months or longer, Fair Value	0		0
Twelve months or longer, Unrealized Losses	0		0
Total, Fair Value	421		298
Total, Unrealized Losses	5		3
Temporarily Impaired Securities | US States and Political Subdivisions Debt Securities [Member]
Investments, Unrealized Loss Position [Line Items]
Less than twelve months, Fair Value	1		1
Less than twelve months, Unrealized Losses	0		0
Twelve months or longer, Fair Value	26		24
Twelve months or longer, Unrealized Losses	2		5
Total, Fair Value	27		25
Total, Unrealized Losses	2		5
Temporarily Impaired Securities | Mortgage-backed Securities, Issued by US Government Sponsored Enterprises
Investments, Unrealized Loss Position [Line Items]
Less than twelve months, Fair Value	4,472		1,212
Less than twelve months, Unrealized Losses	40		3
Twelve months or longer, Fair Value	0		0
Twelve months or longer, Unrealized Losses	0		0
Total, Fair Value	4,472		1,212
Total, Unrealized Losses	40		3
Temporarily Impaired Securities | Asset-backed Securities [Member]
Investments, Unrealized Loss Position [Line Items]
Less than twelve months, Fair Value	0		0
Less than twelve months, Unrealized Losses	0		0
Twelve months or longer, Fair Value	14		13
Twelve months or longer, Unrealized Losses	1		2
Total, Fair Value	14		13
Total, Unrealized Losses	1		2
Other Than Temporarily Impaired Securities [Member]
Investments, Unrealized Loss Position [Line Items]
Less than twelve months, Fair Value	0		0
Less than twelve months, Unrealized Losses	0		0
Twelve months or longer, Fair Value	1	[1]	3	[1]
Twelve months or longer, Unrealized Losses	0		1	[1]
Total, Fair Value	1	[1]	3	[1]
Total, Unrealized Losses	0		1	[1]
Other Than Temporarily Impaired Securities [Member] | Asset-backed Securities [Member]
Investments, Unrealized Loss Position [Line Items]
Less than twelve months, Fair Value	0		0
Less than twelve months, Unrealized Losses	0		0
Twelve months or longer, Fair Value	1	[1]	3	[1]
Twelve months or longer, Unrealized Losses	0		1	[1]
Total, Fair Value	1	[1]	3	[1]
Total, Unrealized Losses	0		1	[1]
Impaired Securities
Investments, Unrealized Loss Position [Line Items]
Less than twelve months, Fair Value	4,894		1,511
Less than twelve months, Unrealized Losses	45		6
Twelve months or longer, Fair Value	41		40
Twelve months or longer, Unrealized Losses	3		8
Total, Fair Value	4,935		1,551
Total, Unrealized Losses	$ 48		$ 14

[1] Includes OTTI securities for which credit losses have been recorded in earnings in current or prior periods.