Earnings Per Share (Basic and Diluted Earnings) (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Earnings Per Share [Abstract]
Net income	$ 15,975	$ 7,799	$ 25,596	$ (26,432)	$ 67,874	$ 144,883	$ 140,204	$ 122,641	$ 22,938	$ 475,602	$ 445,374
Weighted average number of common shares outstanding									29,265	31,092	31,038
Dilutive stock awards									505	402	474
Diluted weighted average number of common shares outstanding									29,770	31,494	31,512
Basic earnings (loss) per share	$ 0.55	$ 0.27	$ 0.88	$ (0.88)	$ 2.19	$ 4.65	$ 4.51	$ 3.95	$ 0.78	$ 15.30	$ 14.35
Diluted earnings (loss) per share	$ 0.55	$ 0.27	$ 0.87	$ (0.88)	$ 2.17	$ 4.59	$ 4.45	$ 3.88	$ 0.77	$ 15.10	$ 14.13