Goodwill and other intangible assets - Additional Information (Detail) (USD $)	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011
Goodwill and Intangible Assets Disclosure [Line Items]
Indefinite Lived Intangible Assets	$ 6,000,000	$ 6,000,000
Amortization Of Intangible Assets	2,593,000	2,255,000
Fully Amortized Intangibles [Member]
Goodwill and Intangible Assets Disclosure [Line Items]
Finite-Lived Core Deposits, Gross	3,000,000
Finite-Lived Customer Relationships, Gross	$ 600,000