Income Taxes - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended			9 Months Ended
	Sep. 30, 2012	Jun. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Dec. 31, 2011
Income Taxes [Line Items]
Total income tax expense (benefit), rate	18.90%	19.80%	28.60%
Combined federal and state statutory tax rate				37.20%
Federal net deferred tax asset	$ 97		$ 62	$ 97	$ 111
State deferred tax liability	$ 23		$ 29	$ 23	$ 19