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, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Earnings Per Share [Abstract]
Net income (loss)	$ 7,660	$ 49,161	$ 9,233	$ 37,600	$ 5,256	$ (15,370)	$ 32,711	$ 53,641	$ 103,654	$ 76,238	$ (175,656)
Basic weighted-average shares outstanding									119,860	124,448	127,794
Dilutive impact of stock options and restricted shares									1,162	696
Diluted weighted-average shares outstanding									121,022	125,144	127,794
Net income (loss) per common share:
Basic									$ 0.86	$ 0.61	$ (1.37)
Diluted									$ 0.86	$ 0.61	$ (1.37)