Income Taxes - Uncertain Tax Positions (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Uncertain Tax Positions
Balance at beginning of year	$ 1,101	$ 343	$ 161
Increases for prior positions	97	37	0
Increases for current year positions	943	721	182
Other increases	0	0	0
Decreases due to settlements	0	0	0
Expiration of the statute of limitations for the assessment of taxes	0	0	0
Other decreases	0	0	0
Balance at end of year	$ 2,141	$ 1,101	$ 343