Income Taxes - Additional Information (Details) - USD ($)	3 Months Ended	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Operating Loss Carryforwards [Line Items]
Federal corporate tax rate			34.00%	34.00%	34.00%
Reduction in deferred tax assets due to change in tax rate			$ 30,019,000
Tax expense	$ 0		561,000	$ 452,000	$ 109,000
Provisional amount related to the re-measurement of certain deferred tax assets and liabilities			30,000,000
Provisional amount related to the one-time transition tax on the mandatory deemed repatriation of foreign earnings			695,000
Cumulative foreign earnings			5,500,000
Net operating loss carryforwards, US	207,900,000		$ 207,900,000
Change in effective income tax rate, effort of Tax Act			(2.24%)
Accrued income tax for interest or penalties	$ 0		$ 0	$ 0	$ 0
Earliest Tax Year
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards expiration year			2028
Open tax year for review by taxing authorities			2011
Latest Tax Year
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards expiration year			2036
Open tax year for review by taxing authorities			2017
Scenario, Forecast
Operating Loss Carryforwards [Line Items]
Federal corporate tax rate		21.00%