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Income Taxes - Additional Information (Detail) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Tax Credit Carryforward [Line Items]      
CARES Act Tax Benefit $ 7.0    
Effective Income Tax Rate Reconciliation, Nondeductible Expense, Impairment Losses, Amount 0.0 $ 0.0 $ 41.0
Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Amount 7.0 0.0 0.0
Deferred Tax Assets, Valuation Allowance 5.4 5.3  
Change in valuation allowance 0.1 2.3 0.0
Unrecognized tax benefits , if recognized, would favorably impact effective tax rate 6.4 3.5 $ 2.8
Accrued interest related to tax obligations 0.1    
Federal and state credit carryforwards 16.2 10.3  
Investment Tax Credit 0.3 1.3  
Deferred Income Tax Assets, Net [1] 2.1 3.8  
Deferred Income Tax Liabilities, Net (158.1) $ (121.3)  
Significant Change in Unrecognized Tax Benefits is Reasonably Possible, Amount of Unrecorded Benefit $ 1.5    
Deferred Tax Assets, Valuation Allowance Expirations [Domain]      
Tax Credit Carryforward [Line Items]      
Valuation Allowance, Deferred Tax Asset, Explanation of Change 0.8 million    
Valuation allowances, State Tax Credits [Member]      
Tax Credit Carryforward [Line Items]      
Change in valuation allowance $ (8.8)    
[1] Included in "Other assets, net" on our accompanying December 31, 2020 and 2019 Consolidated Balance Sheets.