Earnings Per Share (details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended																				12 Months Ended
	Dec. 31, 2013		Sep. 30, 2013		Jun. 30, 2013		Mar. 31, 2013		Dec. 31, 2012		Sep. 30, 2012		Jun. 30, 2012		Mar. 31, 2012		Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Earnings Per Share [Abstract]
Net income	$ 29,006		$ (8,233)		$ 2,126		$ 37,639		$ 28,147	[1]	$ (10,879)	[1]	$ 1,227	[1]	$ 40,284	[1]	$ 28,766	$ (8,527)	$ 1,985	$ 40,820	$ 60,538	$ 58,779	$ 63,044
Average common shares outstanding - basic																					26,974	26,831	26,687
Additional shares for stock based compensation plans																					53	76	57
Average common shares outstanding - diluted																					27,027	26,907	26,744
Earnings per share of common stock - basic	$ 1.07	[2]	$ (0.31)	[2]	$ 0.08	[2]	$ 1.40	[2]	$ 1.05	[1],[2]	$ (0.41)	[1],[2]	$ 0.05	[1],[2]	$ 1.50	[1],[2]	$ 1.08	$ (0.32)	$ 0.07	$ 1.53	$ 2.24	$ 2.19	$ 2.36
Earnings per share of common stock - diluted	$ 1.07	[2]	$ (0.31)	[2]	$ 0.08	[2]	$ 1.40	[2]	$ 1.04	[1],[2]	$ (0.41)	[1],[2]	$ 0.05	[1],[2]	$ 1.50	[1],[2]	$ 1.07	$ (0.32)	$ 0.07	$ 1.53	$ 2.24	$ 2.18	$ 2.36
Antidilutive shares excluded from computation of earnings per share amount																					26	1	2

[1] Prior period balances have been adjusted for a prior period error identified during the first quarter of 2013. See Note 16 for reconciliation to amounts previously reported.

[2] 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.