Fair value measurements - Summary of Level 3 financial instruments (Details) - Level 3 - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Fair value measurements
Fair value	$ 237	$ 77
Fair value of collateral | Residential loan
Fair value measurements
Fair value	$ 192
Appraised value, selling cost	7.00%
Fair value of collateral | Commercial loans
Fair value measurements
Fair value	$ 45
Fair value of collateral | Home equity line of credit
Fair value measurements
Fair value		$ 77
Appraised value, selling cost		7.00%
Fair value of collateral | Real estate acquired in settlement of loans
Fair value measurements
Fair value		$ 186
Appraised value, selling cost		7.00%