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, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Earnings Per Common Share [Abstract]
Net income to common shareholders	$ 83,878	$ 58,902	$ 227,530	$ 160,183
Net income allocated to participating securities - basic and diluted	1,544	1,244	4,238	3,566
Net income allocated to common shareholders - basic and diluted	$ 82,334	$ 57,658	$ 223,292	$ 156,617
Weighted-average common shares - basic	85,348	84,749	85,298	84,577
Dilutive potential common shares	191	231	184	241
Weighted-average common shares - diluted	85,539	84,980	85,482	84,818
Earnings per common share: Basic	$ 0.96	$ 0.68	$ 2.62	$ 1.85
Earnings per common share: Diluted	$ 0.96	$ 0.68	$ 2.61	$ 1.85