Income Taxes - Reconciliation of Unrecognized Income Tax Benefits (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Balance at beginning of year	$ 3,982	$ 4,761
Additions based on tax positions related to the current year	1,158	1,415
Additions based on tax positions related to prior years	1,244	0
Reductions for tax positions of prior years	(5,543)	(1,360)
Reductions for expired statute of limitations	0	(834)
Balance at end of year	$ 841	$ 3,982