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	6 Months Ended
Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Assets, Fair Value Disclosure [Abstract]
Beginning Balance	$ 124,100	$ 149,383	$ 120,068	$ 153,058
Gains (Losses) included in earnings	(6,260)	(7,646)	(5,757)	(12,886)
Gains (losses) included in OCI	4	(1)	(1)	(6)
Additions	3,196	1,473	6,833	3,109
Settlements	(24)	(49)	(127)	(115)
Ending Balance	121,016	143,160	121,016	143,160
Changes in unrealized gains (losses) included in earnings relating to assets still held	(2,031)	(4,946)	3,135	(7,377)
Debt securities available-for-sale \| Mortgage Backed Securities
Assets, Fair Value Disclosure [Abstract]
Beginning Balance	934	1,177	1,014	1,182
Gains (Losses) included in earnings	0	0	0	0
Gains (losses) included in OCI	4	(1)	(1)	(6)
Additions	0	0	0	0
Settlements	0	(25)	(75)	(25)
Ending Balance	938	1,151	938	1,151
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	251	467	278	530
Gains (Losses) included in earnings	0	(1)	0	0
Gains (losses) included in OCI	0	0	0	0
Additions	23	0	24	2
Settlements	(24)	(24)	(52)	(90)
Ending Balance	250	442	250	442
Changes in unrealized gains (losses) included in earnings relating to assets still held	0	(1)	0	1
Trading account debt securities \| Other
Assets, Fair Value Disclosure [Abstract]
Beginning Balance	372	428	381	440
Gains (Losses) included in earnings	(11)	(5)	(20)	(17)
Gains (losses) included in OCI	0	0	0	0
Additions	0	0	0	0
Settlements	0	0	0	0
Ending Balance	361	423	361	423
Changes in unrealized gains (losses) included in earnings relating to assets still held	5	2	9	8
Mortgage Servicing Rights
Assets, Fair Value Disclosure [Abstract]
Beginning Balance	122,543	147,311	118,395	150,906
Gains (Losses) included in earnings	(6,249)	(7,640)	(5,737)	(12,869)
Gains (losses) included in OCI	0	0	0	0
Additions	3,173	1,473	6,809	3,107
Settlements	0	0	0	0
Ending Balance	119,467	141,144	119,467	141,144
Changes in unrealized gains (losses) included in earnings relating to assets still held	$ (2,036)	$ (4,947)	$ 3,126	$ (7,386)

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- Definition

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
-Name Accounting Standards Codification
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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

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
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- Definition

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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