Earnings Per Common Share (Computation of Earnings Per Common Share) (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Earnings Per Share [Abstract]
Net income to common shareholders	$ 79,164	$ 72,475
Net income allocated to participating securities - basic and diluted	1,337	1,366
Net income allocated to common shareholders - basic and diluted	$ 77,827	$ 71,109
Weighted-average common shares - basic	85,688	85,241
Dilutive potential common shares	112	182
Weighted-average common shares - diluted	85,800	85,423
Earnings per common share: Basic	$ 0.91	$ 0.83
Earnings per common share: Diluted	$ 0.91	$ 0.83