Earnings Per Common Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended												12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Earnings Per Share [Abstract]
Net income	$ 73,039	$ 70,397	$ 68,988	$ 71,596	$ 9,446	$ 40,459	$ 52,014	$ 47,604	$ 47,168	$ 11,484	$ 25,775	$ 23,605	$ 284,020	$ 149,523	$ 108,032
Net income allocated to participating securities													186	495	166
Net income allocated to common shareholders													$ 283,834	$ 149,028	$ 107,866
Weighted average common shares - issued (in shares)													71,385,000	71,103,000	49,091,000
Average unvested restricted share awards (in shares)													(47,000)	(238,000)	(139,000)
Weighted average common shares outstanding - basic (in shares)													71,338,000	70,865,000	48,952,000
Effect of dilutive securities
Weighted average common stock equivalents (in shares)													687,000	648,000	651,000
Weighted average common shares outstanding - diluted (in shares)													72,025,000	71,513,000	49,603,000
EPS available to common shareholders
Basic earnings per common share (usd per share)	$ 1.02	$ 0.99	$ 0.97	$ 1.01	$ 0.13	$ 0.57	$ 0.73	$ 0.67	$ 0.67	$ 0.23	$ 0.67	$ 0.61	$ 3.98	$ 2.11	$ 2.21
Diluted earnings per common share (usd per share)	$ 1.01	$ 0.98	$ 0.96	$ 0.99	$ 0.13	$ 0.56	$ 0.73	$ 0.67	$ 0.66	$ 0.23	$ 0.67	$ 0.60	3.94	2.08	2.17
Schedule Of Earnings Per Share By Common Class [Line Items]
Average stock valuation (usd per share)													$ 53.70	$ 50.26	$ 40.58
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)													61,298	107,272	0