Income Taxes - Additional information (Detail) - USD ($)		12 Months Ended
	Jul. 09, 2015	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2013
Income Tax Disclosure [Abstract]
Income tax expense (benefit)		$ 0	$ 0	$ 0
Operating Loss Carryforwards [Line Items]
Tax Cuts And Jobs Act, Change in Tax Rate, deferred tax assets, decrease		21,500,000
Tax Cuts And Jobs Act Of 2017, provisional income tax expense		0
Investor
Operating Loss Carryforwards [Line Items]
Stock issued during period (in shares)	6,459,948
Investor ownership percentage of company	60.20%
U.S. federal
Operating Loss Carryforwards [Line Items]
Operating loss carryforwards		144,000,000
Tax credit carryforward		4,156,000
Operating loss carryforward limitations on use		124,800,000
Tax credit carryforward limitiations on use		3,650,000
State and Local Jurisdiction
Operating Loss Carryforwards [Line Items]
Operating loss carryforwards		89,800,000
Tax credit carryforward		304,000
Operating loss carryforward limitations on use		73,400,000
Tax credit carryforward limitiations on use		324,000
Latest Tax Year | U.S. federal
Operating Loss Carryforwards [Line Items]
Operating loss carryforwards		139,700,000
No Expiration | U.S. federal
Operating Loss Carryforwards [Line Items]
Operating loss carryforwards		$ 4,300,000