Earnings Per Share (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended																12 Months Ended
	Dec. 31, 2014		Sep. 30, 2014		Jun. 30, 2014		Mar. 31, 2014		Dec. 31, 2013		Sep. 30, 2013		Jun. 30, 2013		Mar. 31, 2013		Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Earnings Per Share [Abstract]
Net income	$ 28,470		$ (8,733)		$ 1,071		$ 37,884		$ 29,006		$ (8,233)		$ 2,126		$ 37,639		$ 58,692	$ 60,538	$ 58,779
Average common shares outstanding - basic																	27,164	26,974	26,831
Additional shares for stock based compensation plans																	59	53	76
Average common shares outstanding - diluted																	27,223	27,027	26,907
Earnings per share of common stock - basic	$ 1.05	[1]	$ (0.32)	[1]	$ 0.04	[1]	$ 1.40	[1]	$ 1.07	[1]	$ (0.31)	[1]	$ 0.08	[1]	$ 1.40	[1]	$ 2.16	$ 2.24	$ 2.19
Earnings per share of common stock - diluted	$ 1.04	[1]	$ (0.32)	[1]	$ 0.04	[1]	$ 1.40	[1]	$ 1.07	[1]	$ (0.31)	[1]	$ 0.08	[1]	$ 1.40	[1]	$ 2.16	$ 2.24	$ 2.18
Antidilutive shares excluded from computation of earnings per share amount																	18	26	1

[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.