Gross Carrying Amounts and Accumulated Amortization of Intangible Assets and Goodwill (Detail) (USD $) In Thousands, unless otherwise specified	9 Months Ended	12 Months Ended
	Sep. 30, 2012	Dec. 31, 2011
Goodwill And Intangible Assets [Line Items]
Net Carrying Amount	3,641	4,734
Goodwill gross	81,892	76,816
Goodwill	81,892	76,816
Core Deposits
Goodwill And Intangible Assets [Line Items]
Weighted average useful life	1 year 6 months	2 years 3 months 18 days
Other Intangible Assets
Goodwill And Intangible Assets [Line Items]
Weighted average useful life	3 years 4 months 24 days	3 years 6 months
Amortization of Intangible Assets
Goodwill And Intangible Assets [Line Items]
Gross Carrying Amount	18,327	18,017
Accumulated Amortization	(14,686)	(13,283)
Net Carrying Amount	3,641	4,734
Amortization of Intangible Assets | Core Deposits
Goodwill And Intangible Assets [Line Items]
Gross Carrying Amount	9,716	9,716
Accumulated Amortization	(7,617)	(6,575)
Net Carrying Amount	2,099	3,141
Amortization of Intangible Assets | Other Intangible Assets
Goodwill And Intangible Assets [Line Items]
Gross Carrying Amount	8,611	8,301
Accumulated Amortization	(7,069)	(6,708)
Net Carrying Amount	1,542	1,593