Income Taxes - Total Balance of Unrecognized Tax Benefits (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Unrecognized tax benefits at beginning of the period	$ 2,597	$ 1,878	$ 1,732
Gross increases to tax positions in prior periods	2	210
Gross increases to tax positions in current periods	307	509	146
Unrecognized tax benefits at end of the period	$ 2,906	$ 2,597	$ 1,878