Earnings Per Common Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Earnings Per Share [Abstract]
Net income	$ 70,397	$ 40,459	$ 210,981	$ 140,077
Net income allocated to participating securities	40	129	144	566
Net income allocated to common shareholders	$ 70,357	$ 40,330	$ 210,837	$ 139,511
Weighted average common shares - issued (in shares)	71,426,000	71,139,000	71,365,000	71,077,000
Average unvested restricted share awards (in shares)	(41,000)	(228,000)	(49,000)	(290,000)
Weighted average common shares outstanding - basic (in shares)	71,385,000	70,911,000	71,316,000	70,787,000
Effect of dilutive securities
Weighted average common stock equivalents (in shares)	702,000	594,000	691,000	667,000
Weighted average common shares outstanding - diluted (in shares)	72,087,000	71,505,000	72,007,000	71,454,000
EPS available to common shareholders
Basic earnings per common share (dollars per share)	$ 0.99	$ 0.57	$ 2.96	$ 1.98
Diluted earnings per common share (dollars per share)	0.98	0.56	2.93	1.95
Average stock valuation (dollars per share)	$ 56.59	$ 47.68	$ 56.43	$ 49.07
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)	58,882	125,280	63,014	102,245