Fair value measurements - Summary of Level 3 financial instruments (Details) - Fair Value, Inputs, Level 3 - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2016
Fair value measurements
Fair value	$ 2,881	$ 2,767
Fair value of property or collateral | Residential loan
Fair value measurements
Fair value	$ 731	$ 287
Appraised value, selling cost (as a percent)	7.00%	7.00%
Appraised value, weighted average rate (as a percent)	69.00%	61.00%
Fair value of property or collateral | Residential loan | Minimum
Fair value measurements
Appraised value (as a percent)		42.00%
Fair value of property or collateral | Residential loan | Maximum
Fair value measurements
Appraised value (as a percent)		65.00%
Fair value of property or collateral | Commercial loans
Fair value measurements
Appraised value, weighted average rate (as a percent)	76.00%
Fair value of property or collateral | Home equity line of credit
Fair value measurements
Fair value		$ 12
Appraised value, selling cost (as a percent)		7.00%
Fair value of property or collateral | Real estate acquired in settlement of loans
Fair value measurements
Fair value	$ 93	$ 1,189
Appraised value, selling cost (as a percent)	7.00%	7.00%
Appraised value (as a percent)		100.00%
Appraised value, weighted average rate (as a percent)		100.00%
Sales price | Residential loan
Fair value measurements
Fair value		$ 2,468
Appraised value, weighted average rate (as a percent)		97.00%
Sales price | Residential loan | Minimum
Fair value measurements
Appraised value (as a percent)	50.00%	95.00%
Sales price | Residential loan | Maximum
Fair value measurements
Appraised value (as a percent)	91.00%	100.00%
Sales price | Commercial loans
Fair value measurements
Fair value	$ 2,150
Sales price | Commercial loans | Minimum
Fair value measurements
Appraised value (as a percent)	72.00%
Sales price | Commercial loans | Maximum
Fair value measurements
Appraised value (as a percent)	76.00%