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, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Assets, Fair Value Disclosure [Abstract]
Beginning Balance	$ 155,368	$ 166,508	$ 172,149	$ 170,420
Gains (Losses) included in earnings	(4,855)	(4,202)	(25,883)	(13,148)
Gains (losses) included in OCI	1	(1)	1	1
Additions	2,474	2,953	6,818	8,129
Settlements	(86)	(33)	(185)	(177)
Ending Balance	152,900	165,225	152,900	165,225
Fair Value Assets Measured On Recurring Basis Change In Unrealized Gain Loss	(1,569)	3	(15,979)	20
Liabilities, Fair Value Disclosure [Abstract]
Beginning Balance				(164,858)
Liabilitiy - Gains (Losses) included in earnings				(6,112)
Gains (losses) included in OCI				0
Additions				0
Settlements				170,970
Ending Balance		0		0
Fair Value Liabilities Measured On Recurring Basis Change In Unrealized Gain Loss				0
Contingent consideration
Liabilities, Fair Value Disclosure [Abstract]
Beginning Balance				(164,858)
Liabilitiy - Gains (Losses) included in earnings				(6,112)
Gains (losses) included in OCI				0
Additions				0
Settlements				170,970
Ending Balance		0		0
Fair Value Liabilities Measured On Recurring Basis Change In Unrealized Gain Loss				0
Debt securities available-for-sale \| Mortgage Backed Securities
Assets, Fair Value Disclosure [Abstract]
Beginning Balance	1,235	1,264	1,233	1,288
Gains (Losses) included in earnings	0	0	0	0
Gains (losses) included in OCI	1	(1)	1	1
Additions	0	0	0	0
Settlements	(25)	0	(25)	(26)
Ending Balance	1,209	1,263	1,209	1,263
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	43	43
Gains (Losses) included in earnings	0	0	(1)	0
Gains (losses) included in OCI	0	0	0	0
Additions	0	0	25	0
Settlements	0	0	(41)	0
Ending Balance	26	43	26	43
Fair Value Assets Measured On Recurring Basis Change In Unrealized Gain Loss	0	0	1	0
Trading account debt securities \| CMO
Assets, Fair Value Disclosure [Abstract]
Beginning Balance	618	670	611	529
Gains (Losses) included in earnings	(1)	0	0	6
Gains (losses) included in OCI	0	0	0	0
Additions	1	7	65	260
Settlements	(61)	(33)	(119)	(151)
Ending Balance	557	644	557	644
Fair Value Assets Measured On Recurring Basis Change In Unrealized Gain Loss	1	0	0	6
Trading account debt securities \| Other
Assets, Fair Value Disclosure [Abstract]
Beginning Balance	468	506	485	529
Gains (Losses) included in earnings	(12)	(8)	(29)	(31)
Gains (losses) included in OCI	0	0	0	0
Additions	0	0	0	0
Settlements	0	0	0	0
Ending Balance	456	498	456	498
Fair Value Assets Measured On Recurring Basis Change In Unrealized Gain Loss	5	3	13	14
Mortgage Servicing Rights
Assets, Fair Value Disclosure [Abstract]
Beginning Balance	153,021	164,025	169,777	168,031
Gains (Losses) included in earnings	(4,842)	(4,194)	(25,853)	(13,123)
Gains (losses) included in OCI	0	0	0	0
Additions	2,473	2,946	6,728	7,869
Settlements	0	0	0	0
Ending Balance	150,652	162,777	150,652	162,777
Fair Value Assets Measured On Recurring Basis Change In Unrealized Gain Loss	$ (1,575)	$ 0	$ (15,993)	$ 0

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Amount of unrealized (holding) gain (loss) which is included in the statement of income (or changes in net assets) related to those assets still held at the reporting date for which fair value is measured on a recurring basis using significant unobservable inputs (Level 3).

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Amount of unrealized (holding) gain (loss) which is included in the statement of income (or changes in net assets) related to those liabilities still held at the reporting date for which fair value is measured on a recurring basis using significant unobservable inputs (Level 3).

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Amount of gain (loss) recognized in the income statement for 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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Amount of gain (loss) recognized in other comprehensive income (loss) for 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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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.

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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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Amount of gain (loss) recognized on the income statement for financial instrument classified as a liability measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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Amount of gain (loss) recognized in other comprehensive income (loss) for financial instrument classified as a liability measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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