Commitments and contingencies - Additional Information (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Commitments and Contingencies Disclosure [Abstract]
Total principal amount of loans repurchased or indemnified	$ 2,117	$ 1,543	$ 3,510	$ 2,662