Earnings per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Earnings Per Share [Abstract]
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount			1,000	3,000
Net income	$ 20,979	$ 14,688	$ 38,590	$ 30,714
Undistributed Earnings (Loss) Allocated to Participating Securities, Basic	(211)	(158)	(377)	(333)
Distributed earnings allocated to common stock	7,039	6,797	14,077	13,594
Undistributed earnings allocated to common stock	13,729	7,733	24,136	16,787
Net earnings allocated to common shareholders	$ 20,768	$ 14,530	$ 38,213	$ 30,381
Average shares outstanding (in shares)	15,326,000	15,462,000	15,370,000	15,344,000
Effect of dilutive securities:
Employee stock awards (in shares)	19,000	25,000	20,000	25,000
Average common shares outstanding, diluted (in shares)	15,345,000	15,487,000	15,390,000	15,369,000
Basic earnings per common share (in dollars per share)	$ 1.36	$ 0.94	$ 2.49	$ 1.98
Diluted earnings per share (in dollars per share)	$ 1.35	$ 0.94	$ 2.48	$ 1.98