Earnings Per Share (Weighted-Average Shares of Common Stock Outstanding Used in Calculation of Basic and Diluted Earnings Per Share) (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 (loss)	$ 5,256	$ (15,370)	$ 32,711	$ 53,641	$ 31,019	$ (155,946)	$ 28,305	$ (79,034)	$ 76,238	$ (175,656)	$ (24,295)
Basic weighted-average shares outstanding									124,448	127,794	128,919
Dilutive impact of stock options and restricted shares									696
Diluted weighted-average shares outstanding									125,144	127,794	128,919
Net income (loss) per common share:
Basic									$ 0.61	$ (1.37)	$ (0.19)
Diluted									$ 0.61	$ (1.37)	$ (0.19)