Income Taxes - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended			9 Months Ended
	Sep. 30, 2013	Jun. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Dec. 31, 2012
Income Tax Disclosure [Abstract]
Total income tax expense (benefit), rate	20.10%	26.70%	18.90%
Combined federal and state statutory tax rate				37.20%
Federal net deferred tax asset	$ 219		$ 88	$ 219	$ 129
State deferred tax liability			24		8
State deferred tax asset	9			9
Deferred tax assets, valuation allowance	$ 2		$ 0	$ 2	$ 3