Income Taxes - Additional Information (Detail) - USD ($)	3 Months Ended	12 Months Ended
	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax [Line Items]
Federal income tax expense (benefit)			$ 0	$ 0	$ 0
State income tax expense (benefit)			$ 0	$ 0	$ 0
U.S federal corporate tax rate			34.00%	34.00%	34.00%
Transition tax recognized			$ 0
Net operating loss and tax credit carryforwards ownership interest changing period			3 years
Ownership interest rate of net operating loss and tax credit carryforwards			50.00%
Net operating loss carryforwards, limitation on use			$ 1,500,000
Uncertain tax positions			0	$ 0
Accrued interest or penalties related to uncertain tax positions			0
Increase in the valuation allowance			25,400,000
Tax cuts and jobs act, change in tax rate, decrease in gross deferred tax assets			63,300,000
Tax cuts and jobs act, change in tax rate, decrease in valuation allowance			63,300,000
ASU 2016-09 [Member]
Income Tax [Line Items]
Cumulative-effect adjustment to retained earnings to record a net deferred tax asset	$ 14,900,000
Federal Tax Authority [Member]
Income Tax [Line Items]
Net operating loss carryforwards			$ 418,700,000
Expiration of net operating loss carryforwards			2030
Tax credit carryforwards			$ 11,200,000
Expiration of tax credit carryforwards			2030
State and Local Jurisdiction [Member]
Income Tax [Line Items]
Net operating loss carryforwards			$ 363,600,000
Expiration of net operating loss carryforwards			2019
Tax credit carryforwards			$ 7,400,000
Expiration of tax credit carryforwards			2018
Scenario Plan [Member]
Income Tax [Line Items]
U.S federal corporate tax rate		21.00%
Maximum [Member]
Income Tax [Line Items]
U.S federal corporate tax rate			35.00%