Income Taxes - Narrative (Details) - USD ($) $ in Thousands			3 Months Ended	12 Months Ended
	May 03, 2019	Apr. 23, 2019	Dec. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Jun. 30, 2016
Income Tax Contingency [Line Items]
Income tax refunds				$ 200	$ 6,300
Pretax income (loss) including permanently excludable loss				(54,600)	(47,200)
Provisional adjustment to decrease net deferred tax assets			$ 10,900		$ 10,900
Corresponding net adjustment to deferred tax expense			11,300
Period to carry forward general business credits					20 years
Valuation allowance against net deferred tax assets			31,992	44,405	$ 31,992	$ 17,800
Deferred tax liability			$ 703	647	$ 703
Tax credit carryforwards				5,800
Federal net operating loss carryforwards				13,600
State net operating loss carryforwards				$ 5,400
Effective income tax rate if the impact of the change in valuation of warrants had been excluded				(0.30%)	(0.90%)
Possible decrease in liability for unrecognized tax benefits, excluding interest and penalties				$ 100
Series B Convertible Preferred Stock | Subsequent Event | Share Purchase Agreement, Second Amended And Restated Warrant
Income Tax Contingency [Line Items]
Exercise of private placement warrants (in shares)		4,049,759
Percent ownership of outstanding common stock	51.50%
Private placement warranty liability
Income Tax Contingency [Line Items]
Permanently excludable loss associated with change in valuation of warrants				$ 10,400	$ 4,000