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, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Earnings Per Share [Abstract]
Net income (loss) to common shareholders	$ 79,356	$ 67,807	$ (148,749)	$ 235,248
Net dividends or income allocated to participating securities - basic and diluted	1,435	1,141	1,278	3,980
Net income (loss) allocated to common shareholders - basic and diluted	$ 77,921	$ 66,666	$ (150,027)	$ 231,268
Weighted-average common shares - basic	86,358	86,377	86,614	85,934
Dilutive potential common shares	42	85		76
Weighted-average common shares - diluted	86,400	86,462	86,614	86,010
Earnings per common share: Basic	$ 0.90	$ 0.77	$ (1.73)	$ 2.69
Earnings per common share: Diluted	$ 0.90	$ 0.77	$ (1.73)	$ 2.69