Income Taxes - Unrecognized Tax Benefits (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reconciliation of unrecognized tax benefits
Balance at January 1	$ 8,353	$ 8,195	$ 577
Increases for tax positions taken during the current year	0	361	7,618
(Decreases) for tax positions taken during the current year	(3,500)	0	0
Increases for tax positions taken during prior years	700	0	0
(Decreases) relating to a lapse of the applicable statute of limitations	(212)	(203)	0
Balance at December 31	$ 5,341	$ 8,353	$ 8,195