Earnings Per Common Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Earnings Per Share [Abstract]
Net income	$ 68,988	$ 52,014	$ 140,584	$ 99,618
Net income allocated to participating securities	40	219	105	446
Net income allocated to common shareholders	$ 68,948	$ 51,795	$ 140,479	$ 99,172
Weighted average common shares - issued (in shares)	71,371,000	71,122,000	71,334,000	71,046,000
Average unvested restricted share awards (in shares)	(42,000)	(303,000)	(54,000)	(321,000)
Weighted average common shares outstanding - basic (in shares)	71,329,000	70,819,000	71,280,000	70,725,000
Effect of dilutive securities
Weighted average common stock equivalents (in shares)	697,000	624,000	686,000	704,000
Weighted average common shares outstanding - diluted (in shares)	72,026,000	71,443,000	71,966,000	71,429,000
EPS available to common shareholders
Basic earnings per common share (dollars per share)	$ 0.97	$ 0.73	$ 1.97	$ 1.41
Diluted earnings per common share (dollars per share)	0.96	0.73	1.95	1.39
Average stock valuation (dollars per share)	$ 56.46	$ 47.78	$ 56.35	$ 49.78
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)	59,303	125,782	65,114	90,537