Mortgage servicing rights - Changes in Mortgage Servicing Rights (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Servicing Asset at Fair Value, Amount [Roll Forward]
Carrying value at beginning of period	$ 79,997	$ 75,521	$ 88,829
Capitalization	39,018	47,025	42,151
Mortgage servicing rights acquired from Franklin, at fair value	0	5,111	0
Change in fair value:
Due to pay-offs/pay-downs	(30,583)	(27,834)	(16,350)
Due to change in valuation inputs or assumptions	27,080	(19,826)	(9,949)
Carrying value at end of period	115,512	79,997	75,521
Servicing Asset at Amortized Cost, Disposals	$ 0	$ 0	$ (29,160)