Income Tax - Schedule of Income Tax Computation (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Income Taxes [Line Items]
Tax computed at federal statutory rate	$ 27	$ 39	$ 53
State income taxes, net of federal tax benefit	1	1	3
Income Tax Deferred	$ 28	$ 40	$ 56
Tax computed at federal statutory rate	35.00%	35.00%	35.00%
State income taxes, net of federal tax benefit	1.40%	1.00%	1.80%
Total Federal income tax rate	36.40%	36.00%	36.80%

A ratio calculated by dividing the reported amount of income tax expense attributable to continuing operations for the period by GAAP-basis pretax income from continuing operations.

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The domestic federal statutory tax rate applicable under enacted tax laws to the Company's pretax income from continuing operations for the period. The "statutory" tax rate is the regular tax rate if there are alternative tax systems.

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- Definition

The sum of the current income tax expense or benefit and the deferred income tax expense or benefit pertaining to continuing operations.

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- Definition

The amount of income tax expense or benefit for the period computed by applying the domestic federal statutory tax rates to pretax income from continuing operations.

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