Note Goodwill and other intangible assets (Components of other intangible assets subject to amortization) (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Finite-Lived Intangible Assets [Line Items]
Gross Amount	$ 72,856	$ 72,907
Finite Lived Intangible Assets Accumulated Amortization	45,673	43,398
Finite-Lived Intangible Assets, Net	27,183	29,509
Core deposits
Finite-Lived Intangible Assets [Line Items]
Finite-Lived Core Deposits, Gross	37,224	37,224
Finite Lived Intangible Assets Accumulated Amortization	23,278	22,347
Finite-Lived Intangible Assets, Net	13,946	14,877
Other customer relationship
Finite-Lived Intangible Assets [Line Items]
Finite-Lived Customer Relationships, Gross	35,632	35,683
Finite Lived Intangible Assets Accumulated Amortization	22,395	21,051
Finite-Lived Intangible Assets, Net	$ 13,237	$ 14,632