Earnings Per Share (Computation of Earnings Per Common Share) (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Earnings Per Share [Abstract]
Net income to common shareholders	$ 327,380	$ 323,770	$ 215,632
Net income allocated to participating securities -- basic and diluted	5,546	5,930	4,670
Net income allocated to common shareholders - basic and diluted	$ 321,834	$ 317,840	$ 210,962
Weighted-average common shares - basic	86,488	85,355	84,695
Dilutive potential common shares	111	166	268
Weighted average common shares - diluted	86,599	85,521	84,963
Earnings per common share: Basic	$ 3.72	$ 3.72	$ 2.49
Earnings per common share: Diluted	$ 3.72	$ 3.72	$ 2.48