Components of other intangible assets subject to amortization (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Finite-Lived Intangible Assets [Line Items]
Finite-Lived Core Deposits, Gross	$ 3,000
Finite-Lived Customer Relationships, Gross	4,000
Gross Amount	94,855	100,786
Finite Lived Intangible Assets Accumulated Amortization	44,206	42,945
Finite-Lived Intangible Assets, Net	50,649	57,841
Core Deposits Member
Finite-Lived Intangible Assets [Line Items]
Finite-Lived Core Deposits, Gross	77,885	80,591
Finite Lived Intangible Assets Accumulated Amortization	41,599	38,199
Finite-Lived Intangible Assets, Net	36,286	42,392
Customer Relationships Member
Finite-Lived Intangible Assets [Line Items]
Finite-Lived Customer Relationships, Gross	16,835	19,953
Finite Lived Intangible Assets Accumulated Amortization	2,542	4,643
Finite-Lived Intangible Assets, Net	14,293	15,310
Other Intangible Assets Member
Finite-Lived Intangible Assets [Line Items]
Other Finite-Lived Intangible Assets, Gross	135	242
Finite Lived Intangible Assets Accumulated Amortization	65	103
Finite-Lived Intangible Assets, Net	$ 70	$ 139