Income Taxes - Unrecognized Tax Benefits (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reconciliation of the beginning and ending balance of unrecognized tax benefits
Balance at beginning of year	$ 6,844	$ 7,671	$ 7,960
Increase in unrecognized tax benefits as a result of tax positions taken during a prior period	81	76
Increase in unrecognized tax benefits as a result of tax positions taken during a prior period			(78)
Decreases in unrecognized tax benefits related to settlements with tax authorities			(211)
Reductions to unrecognized tax benefits as a result of a lapse of the applicable statute of limitations	(511)	(903)
Increases in unrecognized tax benefits as a result of tax positions taken during the current period	2,691
Balance at end of year	9,105	6,844	7,671
Recorded as other long-term liability				$ 1,109	$ 1,462	$ 2,142
Recorded as a decrease in deferred tax assets and offsetting valuation allowance				7,996	5,382	5,529
Unrecognized tax benefits	$ 6,844	$ 7,671	$ 7,960	$ 9,105	$ 6,844	$ 7,671