Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Earnings Per Share
Net income for basic and diluted earnings per share	$ 82,337	$ 2,710	$ (10,210)	$ 16,733	$ 12,981	$ 11,365	$ 12,926	$ 13,620	$ 91,570	$ 50,892	$ 31,085
Shares of common stock and common stock equivalents:
Average common shares used in basic computation	33,433	33,859	34,076	34,268	34,521	35,093	35,720	36,081	33,907	35,349	36,788
Effect of dilutive securities (in shares)									908	1,016	1,326
Average common shares used in diluted computation	34,359	34,547	34,076	35,451	35,640	36,026	36,641	37,185	34,815	36,365	38,114
Earnings per share:
Basic (in dollars per share)	$ 2.46	$ 0.08	$ (0.30)	$ 0.49	$ 0.38	$ 0.32	$ 0.36	$ 0.38	$ 2.70	$ 1.44	$ 0.84
Diluted (in dollars per share)	$ 2.40	$ 0.08	$ (0.30)	$ 0.47	$ 0.36	$ 0.32	$ 0.35	$ 0.37	$ 2.63	$ 1.40	$ 0.82
Anti-dilutive equity awards not included in the detailed earnings per share computation
Equity awards not included in the diluted earnings per share computation									100	400