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, 2020	Mar. 31, 2019
Earnings Per Share [Abstract]
Net income (loss) to common shareholders	$ (111,033)	$ 79,164
Net dividends or income allocated to participating securities - basic and diluted	427	1,337
Net income (loss) allocated to common shareholders - basic and diluted	$ (111,460)	$ 77,827
Weighted-average common shares - basic	87,186	85,688
Dilutive potential common shares		112
Weighted-average common shares - diluted	87,186	85,800
Earnings per common share: Basic	$ (1.28)	$ 0.91
Earnings per common share: Diluted	$ (1.28)	$ 0.91