Income Taxes - Unrecognized Tax Benefits (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reconciliation of beginning and ending amount of unrecognized tax benefits
Balance at beginning of year	$ 2,131	$ 805
Additions based on tax positions related to prior years		707	$ 690
Additions based on tax positions related to current year	3,283	619	115
Balance at end of year	$ 5,414	$ 2,131	$ 805