Earnings (Loss) Per Common Share (Computation of Earnings (Loss) Per Common Share) (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Earnings Per Share [Abstract]
Net income (loss) to common shareholders	$ (117,072)	$ 88,277	$ (228,105)	$ 167,441
Net dividends or income allocated to participating securities - basic and diluted	422	1,502	849	2,839
Net income (loss) allocated to common shareholders - basic and diluted	$ (117,494)	$ 86,775	$ (228,954)	$ 164,602
Weighted-average common shares - basic	86,301	85,728	86,744	85,708
Dilutive potential common shares		107		102
Weighted-average common shares - diluted	86,301	85,835	86,744	85,810
Earnings per common share: Basic	$ (1.36)	$ 1.01	$ (2.64)	$ 1.92
Earnings per common share: Diluted	$ (1.36)	$ 1.01	$ (2.64)	$ 1.92