Schedule I - Parent Company Financial Information (Condensed Statements of Comprehensive Income) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended									12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012		Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Condensed Financial Statements, Captions [Line Items]
Net income	$ 192	$ 426	$ 356	[1]	$ 248	$ 199	$ 445	$ 460	$ 315	$ 1,222	$ 1,419	$ 1,099
Other comprehensive income:
Gross unrealized investment gain										164	299	129
Effect of income taxes										(60)	(109)	(47)
Total unrealized investment gain, net of tax										104	190	82
Reclassification adjustment for net realized gains included in net income										(33)	(11)	(6)
Effect of income taxes										12	4	2
Total reclassification adjustment, net of tax										(21)	(7)	(4)
Other comprehensive income, net of tax										83	183	78
Comprehensive income										1,305	1,602	1,177
Parent Company
Condensed Financial Statements, Captions [Line Items]
Net income										1,222	1,419	1,099
Other comprehensive income:
Gross unrealized investment gain										164	299	129
Effect of income taxes										(60)	(109)	(47)
Total unrealized investment gain, net of tax										104	190	82
Reclassification adjustment for net realized gains included in net income										(33)	(11)	(6)
Effect of income taxes										12	4	2
Total reclassification adjustment, net of tax										(21)	(7)	(4)
Other comprehensive income, net of tax										83	183	78
Comprehensive income										$ 1,305	$ 1,602	$ 1,177

[1] Includes an expense of $46 million ($29 million after tax, or $0.18 per diluted common share) for a litigation settlement associated with our military services business.