Income Tax - Reconciliation of Income Tax Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Reconciliation (Amount)
Taxes at statutory rate	$ 19,269	$ 21,318	$ 21,500
Increase (reduction) in income taxes resulting from:
State income tax, net of credits and Federal income tax	771	1,541	1,353
Employee compensation and benefit plans, net	(211)	(654)	(630)
Tax exempt interest income	(167)	(230)	(244)
Benefit due to enactment of federal tax reform	(4,903)	0	0
Other, net	500	168	315
Total income tax expense	$ 15,259	$ 22,143	$ 22,294
Income Tax Reconciliation (Percent)
Taxes at statutory rate	35.00%	35.00%	35.00%
Increase (reduction) in income taxes resulting from:
State income tax, net of credits and Federal income tax	1.40%	2.50%	2.20%
Employee compensation and benefit plans, net	(0.40%)	(1.00%)	(1.00%)
Tax exempt interest income	(0.30%)	(0.40%)	(0.40%)
Benefit due to enactment of federal tax reform	(8.90%)	0.00%	0.00%
Other, net	0.90%	0.30%	0.50%
Income tax expense	27.70%	36.40%	36.30%