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, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Earnings Per Share [Abstract]
Net income (loss)	$ 16,144	$ 49,161	$ (9,702)	$ 95,994
Basic weighted-average shares outstanding	114,597	119,746	115,378	120,745
Dilutive impact of stock options and restricted shares	1,517	1,280		982
Diluted weighted-average shares outstanding	116,114	121,026	115,378	121,727
Net income (loss) per common share
Basic	$ 0.14	$ 0.41	$ (0.08)	$ 0.80
Diluted	$ 0.14	$ 0.41	$ (0.08)	$ 0.79