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		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Earnings Per Share [Abstract]
Net (loss) income	$ (57,391)	$ 9,233	$ (25,846)	$ 46,832
Basic weighted-average shares outstanding	115,064	121,455	115,775	121,253
Dilutive impact of stock options and restricted shares		982		834
Diluted weighted-average shares outstanding	115,064	122,437	115,775	122,087
Net (loss) income per common share
Basic	$ (0.50)	$ 0.08	$ (0.22)	$ 0.39
Diluted	$ (0.50)	$ 0.08	$ (0.22)	$ 0.38