Earnings Per Common Share (Computation of Earnings Per Common Share) (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Earnings Per Share [Abstract]
Net income to common shareholders	$ 88,277	$ 71,177	$ 167,441	$ 143,652
Net income allocated to participating securities - basic and diluted	1,502	1,328	2,839	2,694
Net income allocated to common shareholders - basic and diluted	$ 86,775	$ 69,849	$ 164,602	$ 140,958
Weighted-average common shares - basic	85,728	85,305	85,708	85,273
Dilutive potential common shares	107	178	102	178
Weighted-average common shares - diluted	85,835	85,483	85,810	85,451
Earnings per common share: Basic	$ 1.01	$ 0.82	$ 1.92	$ 1.65
Earnings per common share: Diluted	$ 1.01	$ 0.82	$ 1.92	$ 1.65