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Income Taxes - Additional Information (Detail) (USD $)
In Millions, unless otherwise specified
 3 Months Ended 6 Months Ended
Jun. 30, 2013
Jun. 30, 2012
Jun. 30, 2013
Jun. 30, 2012
Dec. 31, 2012
Income Tax [Line Items]          
Income tax benefit (expense) $ 101.4 $ (0.3) $ 101.2 $ (1.0)  
Effective tax rate (618.30%) 3.00% (652.90%) 7.30%  
Decrease in total deferred tax asset valuation allowances     102.4    
Total deferred tax asset valuation allowances 6.7   6.7    
Unrecognized tax benefits 4.2   4.2   4.0
Unrecognized tax benefits that, if recognized, would reduce the annual effective tax rate 4.2   4.2   3.9
Unrecognized tax benefits that, interests 0.2   0.2    
Unrecognized tax benefits that, penalties 0.1   0.1    
Unrecognized tax benefits expected decrease $ 1.6   $ 1.6