Earnings Per Common Share (Computation of Earnings Per Common Share) (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Earnings Per Share [Abstract]
Net income to common shareholders	$ 67,807	$ 83,878	$ 235,248	$ 227,530
Net income allocated to participating securities - basic and diluted	1,141	1,544	3,980	4,238
Net income allocated to common shareholders - basic and diluted	$ 66,666	$ 82,334	$ 231,268	$ 223,292
Weighted-average common shares - basic	86,377	85,348	85,934	85,298
Dilutive potential common shares	85	191	76	184
Weighted-average common shares - diluted	86,462	85,539	86,010	85,482
Earnings per common share: Basic	$ 0.77	$ 0.96	$ 2.69	$ 2.62
Earnings per common share: Diluted	$ 0.77	$ 0.96	$ 2.69	$ 2.61