Goodwill and other intangible assets - Additional Information (Detail) (USD $)	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013	Dec. 31, 2013
Goodwill and Intangible Assets Disclosure [Line Items]
Indefinite Lived Intangible Assets	$ 6,000,000		$ 6,000,000
Amortization Of Intangible Assets	2,504,000	2,468,000
Subsequent Event Member
Goodwill and Intangible Assets Disclosure [Line Items]
Maximum Expected Goodwill Impairment Loss	220,000,000
Minimum Expected Goodwill Impairment Loss	160,000,000
Fully Amortized Intangibles [Member]
Goodwill and Intangible Assets Disclosure [Line Items]
Finite-Lived Core Deposits, Gross	0
Finite-Lived Customer Relationships, Gross	$ 0