Income Taxes - Narrative (Details) - USD ($)	3 Months Ended	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Provisional income tax expense due to Tax Cuts and Jobs Act of 2017	$ 16,200,000
Operating Loss Carryforwards [Line Items]
Valuation allowance increase (decrease)		$ 5,800,000	$ (9,400,000)	$ 8,500,000
Unrecognized tax benefits	$ 0	$ 0	$ 0
Tax at federal statutory rate		21.00%	35.00%	35.00%
Federal
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards		$ 124,700,000
State
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards		115,300,000
R&D credit
Operating Loss Carryforwards [Line Items]
Credit carryforwards		$ 1,900,000