Income Taxes - Unrecognized Tax Benefits (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Unrecognized tax benefits [Roll Forward]
Balance at January 1,	$ 29,160	$ 30,236
Additions for tax positions related to the current year	184	0
Additions for tax positions related to prior years	0	30
Reductions for the Impact of the Tax Act	(10,859)	0
Reductions for tax positions related to prior years	(24)	(1,106)
Balance at December 31,	$ 18,461	$ 29,160