Income Taxes - Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Deferred tax assets
Net operating loss carryforwards and credits		$ 1,694
Share-based compensation expense		2,203
Other		14
Less: valuation allowance		(3,911)
Total deferred tax assets		0
Deferred tax liabilities
Total deferred tax liabilities		0
Net deferred tax assets (liabilities)		0
Net operating loss carryforwards		8,100
Increase in valuation allowance		$ 3,600
U.S. federal statutory rate, beginning of year		35.00%
Reduction to net deferred tax assets offset		$ 2,600
Forecast
Deferred tax liabilities
U.S. federal statutory rate, beginning of year	21.00%