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Income Taxes - Additional Information (Detail) - USD ($)
 3 Months Ended
Dec. 31, 2017
Mar. 31, 2017
Dec. 31, 2016
Income Tax Contingency [Line Items]      
Tax benefit resulting from the Tax Act $ 171,900,000    
Existing deferred tax assets for which income tax effects are incomplete 3,100,000    
Net operating loss 100,700,000    
Unrecognized tax benefits 0    
Federal [Member]      
Income Tax Contingency [Line Items]      
Net operating loss 764,100,000    
Operating Loss Carryforwards, Valuation Allowance 300,000   $ 600,000
State [Member]      
Income Tax Contingency [Line Items]      
Net operating loss 622,100,000    
ASU 2016-09      
Income Tax Contingency [Line Items]      
Stock-based compensation related to unrealized excess tax benefits   $ 10,600,000  
MONTANA | State [Member]      
Income Tax Contingency [Line Items]      
Operating Loss Carryforwards, Valuation Allowance $ 800,000   $ 800,000