Leadership Changes and Restructuring Activities - Rollforward of Estimated Fair Value Liability Established for Total Severance and Other Related Costs (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Restructuring Cost and Reserve [Line Items]
Non-cash adjustment	$ (2,900)	$ (2,330)	$ (1,453)
Separation And Transition Agreement Or Retirement Agreement [Member]
Restructuring Cost and Reserve [Line Items]
Balance, January 1	964	1,973
Severance and other related expenses		1,055
Accretion	19	59
Cash payments	$ (983)	(1,792)
Non-cash adjustment		(331)
Ending Balance		$ 964	$ 1,973