Mortgage servicing rights - Servicing Income and Expense Included in Mortgage Banking Income (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Servicing income:
Servicing income	$ 22,128	$ 17,677	$ 20,591
Change in fair value of mortgage servicing rights	(47,660)	(26,299)	(2,763)
Change in fair value of derivative hedging instruments	13,286	9,310	(5,910)
Servicing income	(12,246)	688	11,918
Servicing expenses	7,890	6,832	7,675
Net servicing (loss) income	$ (20,136)	$ (6,144)	$ 4,243

All amounts that, per contract, are due to the servicer in exchange for servicing the financial asset and would no longer be received by a servicer if the beneficial owners of the serviced assets (or their trustees or agents) were to exercise their actual or potential authority under the contract to shift the servicing to another servicer. Depending on the servicing contract, those fees may include some or all of the difference between the interest rate collected on the asset being serviced and the rate to be paid to the beneficial owners of the asset.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 50
-Section 50
-Paragraph 2
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=116651436&loc=d3e122596-111746

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration