Income Taxes - Components of Income Tax Expense (Detail) (USD $) In Millions, unless otherwise specified	6 Months Ended	12 Months Ended
Jun. 30, 2013	Jun. 30, 2012	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Current tax expense (benefit) :
Federal			$ 51.3	$ (18.0)	$ (384.0)
State			(5.3)	(22.0)	(15.0)
Total current tax expense (benefit)			46.0	(40.0)	(399.0)
Deferred tax expense (benefit) :
Federal			(49.4)	213.0	568.0
State			(1.8)	2.0	2.0
Total deferred tax expense (benefit)			(51.2)	215.0	570.0
Income tax expense (benefit)	$ 21.3	$ 8.9	$ (5.2)	$ 175.0	$ 171.0

Amount of current federal tax expense (benefit) pertaining to income (loss) from continuing operations.

Amount of current income tax expense (benefit) pertaining to taxable income (loss) from continuing operations.

Amount of current state and local tax expense (benefit) pertaining to income (loss) from continuing operations.

Amount of deferred federal income tax expense (benefit) pertaining to income (loss) from continuing operations.

Amount of deferred income tax expense (benefit) pertaining to income (loss) from continuing operations.

Amount of deferred state and local tax expense (benefit) pertaining to income (loss) from continuing operations.