Earnings Per Common Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Earnings Per Share [Abstract]
Net income	$ 70,170	$ 47,604
Net income allocated to participating securities	64	226
Net income allocated to common shareholders	$ 70,106	$ 47,378
Weighted average common shares - issued (in shares)	71,297,000	70,969,000
Average unvested restricted share awards (in shares)	(66,000)	(341,000)
Weighted average common shares outstanding - basic (in shares)	71,231,000	70,628,000
Effect of dilutive securities
Weighted average common stock equivalents (in shares)	675,000	787,000
Weighted average common shares outstanding - diluted (in shares)	71,906,000	71,415,000
EPS available to common shareholders
Basic earnings per common share (dollars per share)	$ 0.99	$ 0.67
Diluted earnings per common share (dollars per share)	0.97	0.67
Average stock valuation (dollars per share)	$ 56.23	$ 51.81
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)	71,046	0