Income Taxes - Income tax expense (benefit) (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
Federal
Current									$ 7,091	$ 5,801	$ 6,646
Deferred									267	(49)	2,975
Federal income tax									7,358	5,752	9,621
State
Current									1,926	1,265	949
Deferred									72	155	734
State income tax									1,998	1,420	1,683
Deferred tax impairment and statutory rate change											6,210
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