Fair value of financial instruments - Schedule of Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) (Recurring Fair Value Measurements [Member], USD $) In Thousands, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
U.S. Government-agencies	$ 204
U.S. Government-sponsored enterprises	113,045	84,682
State and Municipals:
Taxable	16,698
Tax-exempt	98,081	58,837
Corporate debt securities	4,387
Mortgage-backed securities:
U.S. Government agencies	20,229
U.S. Government-sponsored enterprises	45,974	16,801
Common equity securities	1,097	1,071
Total	299,715	161,391
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
U.S. Government-agencies
U.S. Government-sponsored enterprises
State and Municipals:
Taxable
Tax-exempt
Corporate debt securities
Mortgage-backed securities:
U.S. Government agencies
U.S. Government-sponsored enterprises
Common equity securities	1,097	1,071
Total	1,097	1,071
Significant Observable Inputs (Level 2) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
U.S. Government-agencies	204
U.S. Government-sponsored enterprises	113,045	84,682
State and Municipals:
Taxable	16,698
Tax-exempt	98,081	58,837
Corporate debt securities	4,387
Mortgage-backed securities:
U.S. Government agencies	20,229
U.S. Government-sponsored enterprises	45,974	16,801
Total	298,618	160,320
Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
U.S. Government-agencies
U.S. Government-sponsored enterprises
State and Municipals:
Taxable
Tax-exempt
Corporate debt securities
Mortgage-backed securities:
U.S. Government agencies
U.S. Government-sponsored enterprises
Common equity securities
Total