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Income Taxes - Additional Information (Detail) - USD ($)
 3 Months Ended 12 Months Ended
Dec. 31, 2017
Dec. 31, 2018
Mar. 31, 2017
Income Tax Contingency [Line Items]      
Tax Cuts and Jobs Act, Measurement Period Adjustment, Income Tax Expense (Benefit) $ 1,900,000    
Net increase in valuation allowance   $ 1,700,000  
Unrecognized tax benefits   0  
Federal      
Income Tax Contingency [Line Items]      
Net operating loss   744,200,000  
State      
Income Tax Contingency [Line Items]      
Net operating loss   600,200,000  
ASU 2016-09      
Income Tax Contingency [Line Items]      
Stock-based compensation related to unrealized excess tax benefits     $ 10,600,000
MONTANA | State      
Income Tax Contingency [Line Items]      
Valuation allowance $ 1,200,000 $ 2,900,000