Earnings Per Common Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Earnings Per Share [Abstract]
Net income	$ 62,942	$ 71,596
Net income allocated to participating securities	24	66
Net income allocated to common shareholders	$ 62,918	$ 71,530
Weighted average common shares - issued (in shares)	71,498,000	71,297,000
Average unvested restricted share awards (in shares)	(24,000)	(66,000)
Weighted average common shares outstanding - basic (in shares)	71,474,000	71,231,000
Effect of dilutive securities
Weighted average common stock equivalents (in shares)	667,000	675,000
Weighted average common shares outstanding - diluted (in shares)	72,141,000	71,906,000
EPS available to common shareholders
Basic earnings per common share (dollars per share)	$ 0.88	$ 1.01
Diluted earnings per common share (dollars per share)	0.87	0.99
Average stock valuation (dollars per share)	$ 43.20	$ 56.23
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)	52,244	71,046