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		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Earnings Per Share [Abstract]
Net income (loss)	$ 49,161	$ (15,370)	$ 95,994	$ 70,976
Basic weighted-average shares outstanding	119,746	124,604	120,745	125,041
Dilutive impact of stock options and restricted shares	1,280		982	720
Diluted weighted-average shares outstanding	121,026	124,604	121,727	125,761
Net income (loss) per common share:
Basic	$ 0.41	$ (0.12)	$ 0.80	$ 0.57
Diluted	$ 0.41	$ (0.12)	$ 0.79	$ 0.56