Mortgage servicing rights - Schedule of Servicing Income and Expense Included in Mortgage Banking Income (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Servicing income:
Servicing income	$ 20,591	$ 13,168	$ 12,063
Change in fair value of mortgage servicing rights	(2,763)	(4,023)	0
Change in fair value of derivative hedging instruments	(5,910)	599	0
Total servicing income	11,918	9,744	12,063
Servicing expenses:
Servicing asset amortization	0	0	8,321
Servicing asset impairment	0	0	4,678
Loss on sale of mortgage servicing rights, related hedges and transaction costs on sale	0	249	4,447
Other servicing expenses	7,675	4,896	2,325
Total servicing expenses	7,675	5,145	19,771
Net servicing income (loss)	$ 4,243	$ 4,599	$ (7,708)