Note Fair value measurement (Changes in level 3 assets and liabilities measured at fair value on a recurring basis) (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Assets, Fair Value Disclosure [Abstract]
Beginning Balance	$ 168,594	$ 200,976	$ 170,420	$ 204,959
Gains (Losses) included in earnings	(4,629)	(8,077)	(8,946)	(14,097)
Gains (losses) included in OCI	1		2	10
Additions	2,603	3,252	5,176	6,179
Sales		(160)		(521)
Settlements	(61)	(225)	(144)	(676)
Transfers into Level 3	0	0	0	0
Transfers out of Level 3	0	0	0	(88)
Ending Balance	166,508	195,766	166,508	195,766
Fair Value Assets Measured On Recurring Basis Change In Unrealized Gain Loss	12	(2,885)	17	(3,630)
Liabilities, Fair Value Disclosure [Abstract]
Beginning Balance	(170,970)	(160,543)	(164,858)	(153,158)
Gains (Losses) included in earnings	0	(3,125)	(6,112)	(10,510)
Gains (losses) included in OCI	0		0	0
Additions	0	0	0	0
Sales		0		0
Settlements	170,970	0	170,970	0
Transfer into Level 3	0	0	0	0
Transfer out of Level 3	0	0	0	0
Ending Balance	0	163,668	0	163,668
Fair Value Liabilities Measured On Recurring Basis Change In Unrealized Gain Loss	0	(3,125)	0	(10,510)
Other liabilities | Contingent consideration
Liabilities, Fair Value Disclosure [Abstract]
Beginning Balance	(170,970)	(160,543)	(164,858)	(153,158)
Gains (Losses) included in earnings	0	(3,125)	(6,112)	(10,510)
Gains (losses) included in OCI	0		0	0
Additions	0	0	0	0
Sales		0		0
Settlements	170,970	0	170,970	0
Transfer out of Level 3				0
Ending Balance	0	163,668	0	163,668
Fair Value Liabilities Measured On Recurring Basis Change In Unrealized Gain Loss	0	(3,125)	0	(10,510)
Available-for-sale debt securities | Mortgage Backed Securities
Assets, Fair Value Disclosure [Abstract]
Beginning Balance	1,263	1,289	1,288	1,392
Gains (Losses) included in earnings	0	0	0	0
Gains (losses) included in OCI	1		2	10
Additions	0	0	0	0
Sales		0		0
Settlements	0	0	(26)	(25)
Transfers out of Level 3				(88)
Ending Balance	1,264	1,289	1,264	1,289
Fair Value Assets Measured On Recurring Basis Change In Unrealized Gain Loss	0	0	0	0
Trading account securities | Mortgage Backed Securities
Assets, Fair Value Disclosure [Abstract]
Beginning Balance	43	4,345	43	4,755
Gains (Losses) included in earnings	0	(4)	0	(47)
Gains (losses) included in OCI	0		0	0
Additions	0	168	0	332
Sales		0		(156)
Settlements	0	(175)	0	(550)
Transfers out of Level 3				0
Ending Balance	43	4,334	43	4,334
Fair Value Assets Measured On Recurring Basis Change In Unrealized Gain Loss	0	4	0	(23)
Trading account securities | Collateralized Mortgage Obligations
Assets, Fair Value Disclosure [Abstract]
Beginning Balance	488	1,061	529	1,321
Gains (Losses) included in earnings	6	(1)	6	(5)
Gains (losses) included in OCI	0		0	0
Additions	237	8	253	8
Sales		(160)		(365)
Settlements	(61)	(50)	(118)	(101)
Transfers out of Level 3				0
Ending Balance	670	858	670	858
Fair Value Assets Measured On Recurring Basis Change In Unrealized Gain Loss	6	(2)	6	(6)
Trading account securities | Other
Assets, Fair Value Disclosure [Abstract]
Beginning Balance	519	583	529	602
Gains (Losses) included in earnings	(13)	(26)	(23)	(45)
Gains (losses) included in OCI	0		0	0
Additions	0	0	0	0
Sales		0		0
Settlements	0	0	0	0
Transfers out of Level 3				0
Ending Balance	506	557	506	557
Fair Value Assets Measured On Recurring Basis Change In Unrealized Gain Loss	6	12	11	21
Mortgage Servicing Rights
Assets, Fair Value Disclosure [Abstract]
Beginning Balance	166,281	193,698	168,031	196,889
Gains (Losses) included in earnings	(4,622)	(8,046)	(8,929)	(14,000)
Gains (losses) included in OCI	0		0	0
Additions	2,366	3,076	4,923	5,839
Sales		0		0
Settlements	0	0	0	0
Transfers out of Level 3				0
Ending Balance	164,025	188,728	164,025	188,728
Fair Value Assets Measured On Recurring Basis Change In Unrealized Gain Loss	$ 0	$ (2,899)	$ 0	$ (3,622)