Income Taxes - Unrecognized Tax Benefits (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Reconciliation of the beginning and ending balance of unrecognized tax benefits
Balance at beginning of year	$ 9,105	$ 6,844	$ 7,671
(Decrease) / increase in unrecognized tax benefits as a result of tax positions taken during a prior period		81	76
(Decrease) / increase in unrecognized tax benefits as a result of tax positions taken during a prior period	(132)
Reductions to unrecognized tax benefits as a result of a lapse of the applicable statute of limitations	(543)	(511)	(903)
Increases in unrecognized tax benefits as a result of tax positions taken during the current period	697	2,691
Balance at end of year	9,127	9,105	6,844
Recorded as other long-term liability				$ 676	$ 1,109	$ 1,462
Recorded as a decrease in deferred tax assets and offsetting valuation allowance				8,451	7,996	5,382
Unrecognized tax benefits	$ 9,105	$ 6,844	$ 7,671	$ 9,127	$ 9,105	$ 6,844