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, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Assets, Fair Value Disclosure [Abstract]
Beginning Balance	$ 143,160	$ 155,368	$ 153,058	$ 172,149
Gains (Losses) included in earnings	(20,501)	(4,855)	(33,387)	(25,883)
Gains (losses) included in OCI	(2)	(1)	(8)	1
Additions	2,901	2,474	6,010	6,818
Settlements	(59)	(86)	(174)	(185)
Ending Balance	125,499	152,900	125,499	152,900
Fair Value Assets Measured On Recurring Basis Change In Unrealized Gain Loss	(7,892)	(1,569)	(15,265)	(15,979)
Debt securities available-for-sale \| Mortgage Backed Securities
Assets, Fair Value Disclosure [Abstract]
Beginning Balance	1,151	1,235	1,182	1,233
Gains (Losses) included in earnings	0	0	0	0
Gains (losses) included in OCI	(2)	(1)	(8)	1
Additions	0	0	0	0
Settlements	(25)	(25)	(50)	(25)
Ending Balance	1,124	1,209	1,124	1,209
Fair Value Assets Measured On Recurring Basis Change In Unrealized Gain Loss	0	0	0	0
Trading account debt securities \| Mortgage Backed Securities
Assets, Fair Value Disclosure [Abstract]
Beginning Balance		26		43
Gains (Losses) included in earnings		0		(1)
Gains (losses) included in OCI		0		0
Additions		0		25
Settlements		0		(41)
Ending Balance		26		26
Fair Value Assets Measured On Recurring Basis Change In Unrealized Gain Loss		0		1
Trading account debt securities \| CMO
Assets, Fair Value Disclosure [Abstract]
Beginning Balance	442	618	530	611
Gains (Losses) included in earnings	0	(1)	0	0
Gains (losses) included in OCI	0	0	0	0
Additions	2	1	4	65
Settlements	(34)	(61)	(124)	(119)
Ending Balance	410	557	410	557
Fair Value Assets Measured On Recurring Basis Change In Unrealized Gain Loss	0	1	0	0
Trading account debt securities \| Other
Assets, Fair Value Disclosure [Abstract]
Beginning Balance	423	468	440	485
Gains (Losses) included in earnings	(10)	(12)	(27)	(29)
Gains (losses) included in OCI	0	0	0	0
Additions	0	0	0	0
Settlements	0	0	0	0
Ending Balance	413	456	413	456
Fair Value Assets Measured On Recurring Basis Change In Unrealized Gain Loss	0	5	13	13
Mortgage Servicing Rights
Assets, Fair Value Disclosure [Abstract]
Beginning Balance	141,144	153,021	150,906	169,777
Gains (Losses) included in earnings	(20,491)	(4,842)	(33,360)	(25,853)
Gains (losses) included in OCI	0	0	0	0
Additions	2,899	2,473	6,006	6,728
Settlements	0	0	0	0
Ending Balance	123,552	150,652	123,552	150,652
Fair Value Assets Measured On Recurring Basis Change In Unrealized Gain Loss	$ (7,892)	$ (1,575)	$ (15,278)	$ (15,993)

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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
-Paragraph 2
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258

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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
-Topic 820
-SubTopic 10
-Section 50
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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
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
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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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