Income Taxes - Reconciliation of Income Tax (Expense) Benefit at the Statutory Federal Income Tax Rate and Income Taxes (Details)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Reconciliation of income tax (expense) benefit at the statutory federal income tax rate and income taxes
Federal income tax expense at statutory rate (as a percent)	21.00%	34.00%
Permanent items (as a percent)	1.40%	(8.10%)
Effect of Tax Act (as a percent)		(116.90%)
State income tax, net of federal benefit (as a percent)	6.90%	5.80%
Tax credits (as a percent)	12.20%	29.30%
Change in valuation allowance (as a percent)	(36.60%)	55.80%
Other (as a percent)	(4.30%)
Effective income tax rate (as a percent)	0.60%	(0.10%)