Investment securities - Amortized Cost and Fair Value of Investment Securities Aggregated by Investment Category (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Available for sale, Amortized Cost	$ 274,073	$ 333,294
Available for sale, Gross Unrealized Gains	6,553	6,817
Available for sale, Gross Unrealized Losses	326	525
Available for sale, Fair Value	280,300us-gaap_AvailableForSaleSecurities	339,586us-gaap_AvailableForSaleSecurities
Total Amortized Cost, Held to maturity	14,172us-gaap_HeldToMaturitySecurities	14,665us-gaap_HeldToMaturitySecurities
Held to maturity, Gross Unrealized Gains	590	588
Held to maturity, Gross Unrealized Losses	33	38
Held to maturity, Fair Value	14,729	15,215
State and Municipals Tax Exempt Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Total Amortized Cost, Held to maturity	7,370	7,370
Held to maturity, Gross Unrealized Gains	103	105
Held to maturity, Gross Unrealized Losses	33	38
Held to maturity, Fair Value	7,440	7,437
Mortgage-Backed Securities, U.S. Government Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Available for sale, Amortized Cost	35,339	37,511
Available for sale, Gross Unrealized Gains	154	132
Available for sale, Gross Unrealized Losses	93	167
Available for sale, Fair Value	35,400	37,476
Total Amortized Cost, Held to maturity	96	100
Held to maturity, Gross Unrealized Gains	1	2
Held to maturity, Fair Value	97	102
Mortgage-Backed Securities, U.S. Government-Sponsored Enterprises [Member]
Schedule of Available-for-sale Securities [Line Items]
Available for sale, Amortized Cost	43,471	46,956
Available for sale, Gross Unrealized Gains	286	277
Available for sale, Gross Unrealized Losses	149	226
Available for sale, Fair Value	43,608	47,007
Total Amortized Cost, Held to maturity	6,706	7,195
Held to maturity, Gross Unrealized Gains	486	481
Held to maturity, Fair Value	7,192	7,676
U.S. Government-Sponsored Enterprises State and Municipals [Member]
Schedule of Available-for-sale Securities [Line Items]
Available for sale, Amortized Cost	95,683	95,990
Available for sale, Gross Unrealized Gains	487	337
Available for sale, Gross Unrealized Losses	22	82
Available for sale, Fair Value	96,148	96,245
U.S. Treasury Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available for sale, Amortized Cost		48,393
Available for sale, Gross Unrealized Gains		157
Available for sale, Fair Value		48,550
State and Municipals, Taxable [Member]
Schedule of Available-for-sale Securities [Line Items]
Available for sale, Amortized Cost	16,447	16,490
Available for sale, Gross Unrealized Gains	1,063	943
Available for sale, Gross Unrealized Losses	15	26
Available for sale, Fair Value	17,495	17,407
State and Municipals, Tax-Exempt [Member]
Schedule of Available-for-sale Securities [Line Items]
Available for sale, Amortized Cost	83,133	87,954
Available for sale, Gross Unrealized Gains	4,563	4,971
Available for sale, Gross Unrealized Losses	47	24
Available for sale, Fair Value	$ 87,649	$ 92,901

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

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Amount after other than temporary impairment (OTTI) accretion, of investments in debt securities classified as held-to-maturity.

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Fair value of investments in debt securities classified as held-to-maturity.

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