Mortgage servicing rights - Schedule of Changes in Mortgage Servicing Rights (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Servicing Asset at Fair Value, Amount [Roll Forward]
Carrying value at beginning of period	$ 60,508	$ 66,380	$ 75,521	$ 88,829
Capitalization	18,202	10,387	33,415	30,319
Mortgage servicing rights acquired from Franklin, at fair value	4,850	0	4,850	0
Sales	0	0	0	(29,160)
Change in fair value:
Due to pay-offs/pay-downs	(7,756)	(5,050)	(19,676)	(10,150)
Due to change in valuation inputs or assumptions	581	(5,561)	(22,575)	(13,682)
Carrying value at end of period	$ 71,535	$ 66,156	$ 71,535	$ 66,156