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, 2019	Mar. 31, 2019	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2019	Jun. 30, 2018
Earnings Per Share [Abstract]
Net income (loss)	$ 31,973	$ 25,989	$ (57,391)	$ 31,545	$ 57,962	$ (25,846)
Basic weighted-average shares outstanding	113,205		115,064		112,958	115,775
Dilutive impact of stock options and restricted shares	1,735				1,685
Diluted weighted-average shares outstanding	114,940		115,064		114,643	115,775
Net income per common share:
Basic	$ 0.28		$ (0.50)		$ 0.51	$ (0.22)
Diluted	$ 0.28		$ (0.50)		$ 0.51	$ (0.22)