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, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Assets, Fair Value Disclosure [Abstract]
Beginning Balance	$ 170,164	$ 168,594	$ 172,149	$ 170,420
Gains (Losses) included in earnings	(17,196)	(4,629)	(21,028)	(8,946)
Gains (losses) included in OCI		1	2	2
Additions	2,469	2,603	4,344	5,176
Settlements	(69)	(61)	(99)	(144)
Ending Balance	155,368	166,508	155,368	166,508
Fair Value Assets Measured On Recurring Basis Change In Unrealized Gain Loss	(13,664)	12	(14,408)	17
Liabilities, Fair Value Disclosure [Abstract]
Beginning Balance		(170,970)		(164,858)
Liabilitiy - Gains (Losses) included in earnings		0		(6,112)
Gains (losses) included in OCI		0		0
Additions		0		0
Settlements		170,970		170,970
Ending Balance		0		0
Fair Value Liabilities Measured On Recurring Basis Change In Unrealized Gain Loss		0		0
Contingent consideration
Liabilities, Fair Value Disclosure [Abstract]
Beginning Balance		(170,970)		(164,858)
Liabilitiy - Gains (Losses) included in earnings		0		(6,112)
Gains (losses) included in OCI		0		0
Additions		0		0
Settlements		170,970		170,970
Ending Balance		0		0
Fair Value Liabilities Measured On Recurring Basis Change In Unrealized Gain Loss		0		0
Debt securities available-for-sale \| Mortgage Backed Securities
Assets, Fair Value Disclosure [Abstract]
Beginning Balance	1,235	1,263	1,233	1,288
Gains (Losses) included in earnings	0	0	0	0
Gains (losses) included in OCI		1	2	2
Additions	0	0	0	0
Settlements	0	0	0	(26)
Ending Balance	1,235	1,264	1,235	1,264
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	43	43	43	43
Gains (Losses) included in earnings	(1)	0	(1)	0
Gains (losses) included in OCI		0	0	0
Additions	25	0	25	0
Settlements	(41)	0	(41)	0
Ending Balance	26	43	26	43
Fair Value Assets Measured On Recurring Basis Change In Unrealized Gain Loss	1	0	1	0
Trading account debt securities \| CMO
Assets, Fair Value Disclosure [Abstract]
Beginning Balance	595	488	611	529
Gains (Losses) included in earnings	1	6	1	6
Gains (losses) included in OCI	1	0	0	0
Additions	50	237	64	253
Settlements	(28)	(61)	(58)	(118)
Ending Balance	618	670	618	670
Fair Value Assets Measured On Recurring Basis Change In Unrealized Gain Loss	1	6	1	6
Trading account debt securities \| Other
Assets, Fair Value Disclosure [Abstract]
Beginning Balance	478	519	485	529
Gains (Losses) included in earnings	(10)	(13)	(17)	(23)
Gains (losses) included in OCI		0	0	0
Additions	0	0	0	0
Settlements	0	0	0	0
Ending Balance	468	506	468	506
Fair Value Assets Measured On Recurring Basis Change In Unrealized Gain Loss	5	6	8	11
Mortgage Servicing Rights
Assets, Fair Value Disclosure [Abstract]
Beginning Balance	167,813	166,281	169,777	168,031
Gains (Losses) included in earnings	(17,186)	(4,622)	(21,011)	(8,929)
Gains (losses) included in OCI		0	0	0
Additions	2,394	2,366	4,255	4,923
Settlements	0	0	0	0
Ending Balance	153,021	164,025	153,021	164,025
Fair Value Assets Measured On Recurring Basis Change In Unrealized Gain Loss	$ (13,671)	$ 0	$ (14,418)	$ 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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