Fair value measurement - Changes in level 3 assets and liabilities measured at fair value on a recurring basis (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Assets, Fair Value Disclosure [Abstract]
Beginning Balance	$ 121,016	$ 143,160	$ 120,068	$ 153,058
Gains (Losses) included in earnings	(5,988)	(20,501)	(11,745)	(33,387)
Gains (losses) included in OCI	(5)	(2)	(6)	(8)
Additions	3,083	2,901	9,916	6,010
Settlements	(84)	(59)	(211)	(174)
Ending Balance	118,022	125,499	118,022	125,499
Changes in unrealized gains (losses) included in earnings relating to assets still held	(1,908)	(7,892)	1,226	(15,265)
Debt securities available-for-sale \| Mortgage Backed Securities
Assets, Fair Value Disclosure [Abstract]
Beginning Balance	938	1,151	1,014	1,182
Gains (Losses) included in earnings	0	0	0	0
Gains (losses) included in OCI	(5)	(2)	(6)	(8)
Additions	0	0	0	0
Settlements	(50)	(25)	(125)	(50)
Ending Balance	883	1,124	883	1,124
Changes in unrealized gains (losses) included in earnings relating to assets still held	0	0	0	0
Trading account debt securities \| CMO
Assets, Fair Value Disclosure [Abstract]
Beginning Balance	250	442	278	530
Gains (Losses) included in earnings	0	0	0	0
Gains (losses) included in OCI	0	0	0	0
Additions	4	2	28	4
Settlements	(34)	(34)	(86)	(124)
Ending Balance	220	410	220	410
Changes in unrealized gains (losses) included in earnings relating to assets still held	0	0	(1)	0
Trading account debt securities \| Other
Assets, Fair Value Disclosure [Abstract]
Beginning Balance	361	423	381	440
Gains (Losses) included in earnings	(9)	(10)	(29)	(27)
Gains (losses) included in OCI	0	0	0	0
Additions	0	0	0	0
Settlements	0	0	0	0
Ending Balance	352	413	352	413
Changes in unrealized gains (losses) included in earnings relating to assets still held	4	0	13	13
Mortgage Servicing Rights
Assets, Fair Value Disclosure [Abstract]
Beginning Balance	119,467	141,144	118,395	150,906
Gains (Losses) included in earnings	(5,979)	(20,491)	(11,716)	(33,360)
Gains (losses) included in OCI	0	0	0	0
Additions	3,079	2,899	9,888	6,006
Settlements	0	0	0	0
Ending Balance	116,567	123,552	116,567	123,552
Changes in unrealized gains (losses) included in earnings relating to assets still held	$ (1,912)	$ (7,892)	$ 1,214	$ (15,278)

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Amount of unrealized gain (loss) recognized in income from asset measured at fair value on recurring basis using unobservable input (level 3) and still held.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258

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- Definition

Amount of gain (loss) recognized in income from asset measured at fair value on recurring basis using unobservable input (level 3).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
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- Definition

Amount of gain (loss) recognized in other comprehensive income (OCI) from asset measured at fair value on recurring basis using unobservable input (level 3).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
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- Definition

Amount of purchases of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

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-Publisher FASB
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Amount of settlement of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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Fair value of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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