Income Taxes - Reconciliation between applicable income taxes and statutory federal tax rate (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	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
Tax amount
Taxes at statutory federal income tax rate									$ 8,168	$ 6,938	$ 11,380
Tax effect of tax exempt income									(442)	(365)	(562)
State income taxes, net of federal benefits									1,618	1,399	1,414
Change in federal statutory tax rate											4,818
Deferred tax impairment											1,392
Nondeductible items and other									12	(800)	(928)
Total income tax expense	$ 2,131	$ 2,332	$ 2,869	$ 2,024	$ 1,920	$ 1,951	$ 1,567	$ 1,734	$ 9,356	$ 7,172	$ 17,514
Percentage of pretax income
Taxes at statutory federal income tax rate (as percentage)									21.00%	21.00%	35.00%
Tax effect of tax exempt income (as percentage)									(1.10%)	(1.10%)	(1.70%)
State income taxes, net of federal benefits (as percentage)									4.20%	4.20%	4.30%
Change in federal statutory tax rate (as percentage)											14.10%
Deferred tax impairment (as percentage)											4.30%
Nondeductible items and other (as percentage)										(2.40%)	(2.90%)
Applicable income taxes (as percentage)									24.10%	21.70%	53.10%