Mortgage servicing rights - Schedule of Changes in Mortgage Servicing Rights (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Servicing Asset at Fair Value, Amount [Roll Forward]
Carrying value at beginning of period	$ 62,581	$ 64,031	$ 75,521	$ 88,829
Capitalization	12,267	11,212	20,063	19,932
Sales	0	0	0	(29,160)
Change in fair value:
Due to pay-offs/pay-downs	(7,277)	(3,305)	(11,920)	(5,100)
Due to change in valuation inputs or assumptions	(7,063)	(5,558)	(23,156)	(8,121)
Carrying value at end of period	$ 60,508	$ 66,380	$ 60,508	$ 66,380