Components of other intangible assets subject to amortization (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2012	Dec. 31, 2011	Mar. 31, 2011
Finite-Lived Intangible Assets [Line Items]
Gross Amount	$ 97,864	$ 100,786	$ 85,872
Finite Lived Intangible Assets Accumulated Amortization	42,180	42,945	35,545
Core Deposits Member
Finite-Lived Intangible Assets [Line Items]
Finite-Lived Core Deposits, Gross	77,885	80,591	80,591
Finite Lived Intangible Assets Accumulated Amortization	37,543	38,199	31,912
Customer Relationships Member
Finite-Lived Intangible Assets [Line Items]
Finite-Lived Customer Relationships, Gross	19,791	19,953	5,092
Finite Lived Intangible Assets Accumulated Amortization	4,563	4,643	3,578
Other Intangible Assets Member
Finite-Lived Intangible Assets [Line Items]
Other Finite-Lived Intangible Assets, Gross	188	242	189
Finite Lived Intangible Assets Accumulated Amortization	$ 74	$ 103	$ 55