Investment securities - Fair Value and Unrealized Losses of Investment Securities in Continuous Unrealized Loss Position (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Fair Value	$ 68,948	$ 133,317
Less Than 12 Months, Unrealized Losses	449	1,945
12 Months or More, Fair Value	10,921	3,114
12 Months or More, Unrealized Losses	114	4
Total, Fair Value	79,869	136,431
Total, Unrealized Losses	563	1,949
State and Municipals, Taxable [Member]
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Fair Value		10,621
Less Than 12 Months, Unrealized Losses		126
12 Months or More, Fair Value	544
12 Months or More, Unrealized Losses	26
Total, Fair Value	544	10,621
Total, Unrealized Losses	26	126
State and Municipals, Tax-Exempt [Member]
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Fair Value	4,702	36,471
Less Than 12 Months, Unrealized Losses	23	1,157
12 Months or More, Fair Value	2,423
12 Months or More, Unrealized Losses	39
Total, Fair Value	7,125	36,471
Total, Unrealized Losses	62	1,157
U.S. Government-Sponsored Enterprises State and Municipals [Member]
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Fair Value	21,228	66,391
Less Than 12 Months, Unrealized Losses	33	468
12 Months or More, Fair Value	7,954	3,114
12 Months or More, Unrealized Losses	49	4
Total, Fair Value	29,182	69,505
Total, Unrealized Losses	82	472
Mortgage-Backed Securities, U.S. Government Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Fair Value	20,148	12,978
Less Than 12 Months, Unrealized Losses	167	66
Total, Fair Value	20,148	12,978
Total, Unrealized Losses	167	66
Mortgage-Backed Securities, U.S. Government-Sponsored Enterprises [Member]
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Fair Value	22,870	5,624
Less Than 12 Months, Unrealized Losses	226	40
Total, Fair Value	22,870	5,624
Total, Unrealized Losses	226	40
Corporate Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Fair Value		1,095
Less Than 12 Months, Unrealized Losses		78
Total, Fair Value		1,095
Total, Unrealized Losses		78
Common Stock [Member]
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Fair Value		137
Less Than 12 Months, Unrealized Losses		10
Total, Fair Value		137
Total, Unrealized Losses		$ 10

Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale that have been in a continuous loss position for twelve months or longer.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

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Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale in a continuous loss position.

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This item represents the aggregate fair value of investments in debt and equity securities in an unrealized loss position which are categorized neither as held-to-maturity nor trading securities.

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Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale that have been in a continuous loss position for less than twelve months.

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