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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 |

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