Fair value measurements - Summary of Level 3 financial instruments (Details) - Level 3 $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2019 USD ($)	Dec. 31, 2018 USD ($)
Fair value measurements
Fair value	$ 3,911	$ 77
Home equity line of credit
Fair value measurements
Fair value	$ 199	$ 77
Appraised value, selling cost	7.00%	7.00%
Residential land
Fair value measurements
Fair value	$ 25
Appraised value, selling cost	7.00%
Commercial
Fair value measurements
Fair value	$ 3,687
Real estate acquired in settlement of loans
Fair value measurements
Fair value		$ 186
Appraised value, selling cost		7.00%
Minimum | Discount rate | Commercial
Fair value measurements
Unobservable input value	0.0390
Maximum | Discount rate | Commercial
Fair value measurements
Unobservable input value	0.0680
Weighted Average | Discount rate | Commercial
Fair value measurements
Unobservable input value	0.046