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 [Abstract]
Net Income	$ 29,705		$ (6,685)		$ 2,197		$ 28,486		$ 28,470		$ (8,733)		$ 1,071		$ 37,884		$ 53,703	$ 58,692	$ 60,538
Average common shares outstanding - basic																	27,347	27,164	26,974
Incremental Common Shares Attributable to Dilutive Effect of Share-based Payment Arrangements																	70	59	53
Average common shares outstanding - diluted																	27,417	27,223	27,027
Earnings per share of common stock - basic	$ 1.08	[1]	$ (0.24)	[1]	$ 0.08	[1]	$ 1.04	[1]	$ 1.05	[1]	$ (0.32)	[1]	$ 0.04	[1]	$ 1.40	[1]	$ 1.96	$ 2.16	$ 2.24
Earnings per share of common stock - diluted	$ 1.08	[1]	$ (0.24)	[1]	$ 0.08	[1]	$ 1.04	[1]	$ 1.04	[1]	$ (0.32)	[1]	$ 0.04	[1]	$ 1.40	[1]	$ 1.96	$ 2.16	$ 2.24
Antidilutive shares excluded from computation of earnings per share amount																	12	18	26

[1] Quarterly earnings (loss) per share are based upon the average number of common shares outstanding during each quarter. Variations in earnings between quarterly periods are due primarily to the seasonal nature of our business.