Income Taxes - Summary of Income Tax Contingencies (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Unrecognized tax benefits, opening balance	$ 2,549	$ 1,982	$ 1,736
Gross increases in tax positions taken in a prior year	80	77	187
Gross increases in tax positions taken in the current year	403	856	763
Lapse of applicable statute of limitations	(283)	(366)	(704)
Unrecognized tax benefits, ending balance	$ 2,749	$ 2,549	$ 1,982