Mortgage servicing rights - Schedule of Changes in Mortgage Servicing Rights (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Servicing Asset at Fair Value, Amount [Roll Forward]
Carrying value at beginning of period	$ 66,380	$ 109,449	$ 88,829	$ 76,107
Capitalization	10,387	11,741	30,319	41,555
Sales	0	(39,428)	(29,160)	(39,428)
Change in fair value:
Due to pay-offs/pay-downs	(5,050)	(3,339)	(10,150)	(8,606)
Due to change in valuation inputs or assumptions	(5,561)	1,467	(13,682)	10,262
Carrying value at period end	$ 66,156	$ 79,890	$ 66,156	$ 79,890