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		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Earnings Per Share [Abstract]
Net income (loss) to common shareholders	$ 129,582	$ 79,356	$ 325,472	$ (148,749)
Net dividends or income allocated to participating securities - basic and diluted	2,419	1,435	6,650	1,278
Net income (loss) allocated to common shareholders - basic and diluted	$ 127,163	$ 77,921	$ 318,822	$ (150,027)
Weighted-average common shares - basic	86,834	86,358	86,800	86,614
Dilutive potential common shares	172	42	151
Weighted-average common shares - diluted	87,006	86,400	86,951	86,614
Earnings per common share: Basic	$ 1.46	$ 0.90	$ 3.67	$ (1.73)
Earnings per common share: Diluted	$ 1.46	$ 0.90	$ 3.67	$ (1.73)