Restructuring Charges (Details) - Employee Separation Costs - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Restructuring Cost and Reserve [Line Items]
Costs incurred and charged to expense	$ 2,526	$ 1,489	$ 5,279
Restructuring Reserve [Roll Forward]
Accrued liability beginning balance	209	1,404	449
Costs incurred and charged to expense	2,526	1,489	5,279
Cash payments	(2,491)	(2,684)	(4,324)
Accrued liability ending balance	$ 244	$ 209	$ 1,404