Commitments and contingencies - Additional Information (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Jan. 01, 2020	Jan. 01, 2019
Loss Contingencies [Line Items]
Initial adoption of ASC			$ (25,018)	$ (1,309)
Total principal amount of loans repurchased or indemnified	$ 2,799	$ 1,393
Unfunded Commitments
Loss Contingencies [Line Items]
Initial adoption of ASC			2,947
Retained earnings
Loss Contingencies [Line Items]
Initial adoption of ASC			$ (25,018)	$ (1,309)