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
	Mar. 31, 2021	Mar. 31, 2020
Earnings Per Share [Abstract]
Net income (loss) to common shareholders	$ 107,172	$ (111,033)
Net dividends or income allocated to participating securities - basic and diluted	2,337	427
Net income (loss) allocated to common shareholders - basic and diluted	$ 104,835	$ (111,460)
Weighted-average common shares - basic	86,752	87,186
Dilutive potential common shares	53
Weighted-average common shares - diluted	86,805	87,186
Earnings per common share: Basic	$ 1.21	$ (1.28)
Earnings per common share: Diluted	$ 1.21	$ (1.28)