Earnings per 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]
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount			400	3,000
Net income	$ 20,692	$ 13,932	$ 59,283	$ 44,646
Undistributed Earnings (Loss) Allocated to Participating Securities, Basic	(201)	(154)	(575)	(488)
Distributed earnings allocated to common stock	8,109	6,797	22,186	20,391
Undistributed earnings allocated to common stock	12,382	6,981	36,522	23,767
Net earnings allocated to common shareholders	$ 20,491	$ 13,778	$ 58,708	$ 44,158
Average shares outstanding (in shares)	15,340,000	15,485,000	15,360,000	15,391,000
Effect of dilutive securities:
Employee stock awards (in shares)	18,000	20,000	20,000	24,000
Average common shares outstanding, diluted (in shares)	15,358,000	15,505,000	15,380,000	15,415,000
Basic earnings per common share (in dollars per share)	$ 1.34	$ 0.89	$ 3.82	$ 2.87
Diluted earnings per share (in dollars per share)	$ 1.33	$ 0.89	$ 3.82	$ 2.86