Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	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, 2017	Dec. 31, 2016	Dec. 31, 2015
Basic
Net income available to common shareholders	$ 2,457	$ 17,394	$ 16,926	$ 15,717	$ 15,118	$ 15,138	$ 14,833	$ 14,251	$ 52,494	$ 59,340	$ 58,421
Less: income attributable to unvested stock-based compensation awards									(818)	(912)	(834)
Net earnings allocated to common shareholders									$ 51,676	$ 58,428	$ 57,587
Weighted average shares outstanding, including unvested stock-based compensation awards (in shares)									15,193,438	15,044,733	14,940,274
Less: unvested stock-based compensation awards (in shares)									(243,006)	(232,021)	(212,081)
Weighted average shares outstanding - Basic (in shares)									14,950,432	14,812,712	14,728,193
Diluted
Net earnings allocated to common shareholders									$ 51,676	$ 58,428	$ 57,587
Weighted average shares outstanding - Basic (in shares)									14,950,432	14,812,712	14,728,193
Plus: incremental shares from assumed conversion of stock-based compensation awards (in shares)									122,823	123,519	134,833
Weighted average shares outstanding - Diluted (in shares)									15,073,255	14,936,231	14,863,026
Basic EPS (in dollars per share)	$ 0.16	$ 1.14	$ 1.11	$ 1.04	$ 1.00	$ 1.01	$ 0.99	$ 0.95	$ 3.46	$ 3.94	$ 3.91
Diluted EPS (in dollars per share)	$ 0.16	$ 1.14	$ 1.11	$ 1.03	$ 0.99	$ 1.00	$ 0.98	$ 0.94	$ 3.43	$ 3.91	$ 3.87
Antidilutive securities excluded from computation of earning per share (in shares)									20,789	72,321	108,159