Income Taxes - Provision and Reconciliation to Federal Statutory Corporate Tax Rate (Details) - USD ($) $ in Thousands		3 Months Ended								12 Months Ended
	Jan. 01, 2018	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Current
Federal										$ 3,366	$ 3,752
State										1,479	1,530
Total current										4,845	5,282
Deferred
Federal										1,147	600
State										319	229
Total deferred										1,466	829
Total income tax expense		$ 1,148	$ 1,775	$ 1,415	$ 1,973	$ 1,737	$ 1,268	$ 1,347	$ 1,759	$ 6,311	$ 6,111
Federal statutory corporate tax rate (as a percent)	21.00%									21.00%	21.00%	35.00%
Reconciliation of tax provision
Income tax expense at statutory rate										$ 5,944	$ 5,318
Income tax effect of:
Other tax-exempt income										(263)	(182)
Share-based compensation										(69)	1
Meal and entertainment expenses										82	88
State income taxes, net of federal income tax benefits										1,301	1,246
Tax benefit from the exercise of stock options										(297)	(134)
Tax benefit from tax depreciation study (1)										(402)
Other										15	(226)
Total income tax expense		$ 1,148	$ 1,775	$ 1,415	$ 1,973	$ 1,737	$ 1,268	$ 1,347	$ 1,759	$ 6,311	$ 6,111
Effective income tax rate (as a percent)										22.29%	24.13%
Federal statutory corporate tax rate (as a percent)	21.00%									21.00%	21.00%	35.00%