Earnings Per Common Share (Details) - USD ($) $ / shares in Units, $ 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	$ 69,594	$ 68,988	$ 132,536	$ 140,584
Net income allocated to participating securities	0	40	24	105
Net income allocated to common shareholders	$ 69,594	$ 68,948	$ 132,512	$ 140,479
Weighted average common shares - issued (in shares)	71,554,000	71,371,000	71,526,000	71,334,000
Average unvested restricted share awards (in shares)	0	(42,000)	(12,000)	(54,000)
Weighted average common shares outstanding - basic (in shares)	71,554,000	71,329,000	71,514,000	71,280,000
Effect of dilutive securities
Weighted average common stock equivalents (in shares)	718,000	697,000	693,000	686,000
Weighted average common shares outstanding - diluted (in shares)	72,272,000	72,026,000	72,207,000	71,966,000
EPS available to common shareholders
Basic earnings per common share (dollars per share)	$ 0.97	$ 0.97	$ 1.85	$ 1.97
Diluted earnings per common share (dollars per share)	0.96	0.96	1.84	1.95
Average stock valuation (dollars per share)	$ 41.46	$ 56.46	$ 42.33	$ 56.35
Stock Option
Schedule Of Earnings Per Share By Common Class [Line Items]
Average number of exercisable employee stock option awards that were not included in the computation of diluted earnings per common share (in shares)	41,695	59,303	46,940	65,114