Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Earnings Per Share
Net (loss) income for basic and diluted (loss) income per share	$ (2,418)	$ (46,967)	$ 4,643	$ 5,302	$ 5,716	$ (23,911)	$ (5,217)	$ (2,506)	$ (39,440)	$ (25,918)	$ 91,570
Shares of common stock and common stock equivalents:
Average common shares used in basic computation	32,855	33,105	33,125	32,949	32,607	32,725	33,189	33,106	33,009	32,906	33,907
Effect of dilutive securities (in shares)											908
Average common shares used in diluted computation	32,855	33,105	34,222	34,130	33,988	32,725	33,189	33,106	33,009	32,906	34,815
(Loss) income per share:
Basic (in dollars per share)	$ (0.07)	$ (1.42)	$ 0.14	$ 0.16	$ 0.18	$ (0.73)	$ (0.16)	$ (0.08)	$ (1.19)	$ (0.79)	$ 2.70
Diluted (in dollars per share)	$ (0.07)	$ (1.42)	$ 0.14	$ 0.16	$ 0.17	$ (0.73)	$ (0.16)	$ (0.08)	$ (1.19)	$ (0.79)	$ 2.63
Anti-dilutive equity awards not included in the detailed earnings per share computation
Equity awards not included in the diluted earnings per share computation									0	0	100