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, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Earnings Per Share [Abstract]
Net income (loss) to common shareholders	$ 88,718	$ (117,072)	$ 195,890	$ (228,105)
Net dividends or income allocated to participating securities - basic and diluted	1,894	422	4,231	849
Net income (loss) allocated to common shareholders - basic and diluted	$ 86,824	$ (117,494)	$ 191,659	$ (228,954)
Weighted-average common shares - basic	86,814	86,301	86,783	86,744
Dilutive potential common shares	176		149
Weighted-average common shares - diluted	86,990	86,301	86,932	86,744
Earnings per common share: Basic	$ 1.00	$ (1.36)	$ 2.21	$ (2.64)
Earnings per common share: Diluted	$ 1.00	$ (1.36)	$ 2.20	$ (2.64)