Summary of Amortized Cost and Estimated Fair Value of Available-For-Sale Securities and Held-To-Maturity Securities (Detail) - USD ($) $ in Thousands	9 Months Ended
Sep. 30, 2015	Dec. 31, 2014
Schedule of Trading Securities and Other Trading Assets [Line Items]
Available-for-Sale, Amortized Cost	$ 237,320
Available-for-sale Securities, Gross Unrealized Gains	4,124
Available-for-sale Securities, Gross Unrealized Losses	1,813
Available-for-sale securities, Estimated Fair Value	239,631	$ 254,746
Held-to-Maturity, Amortized Cost	7,233
Held-to-maturity Securities, Gross Unrealized Gains	1,418
Held-to-maturity Securities, Gross Unrealized Losses	0
Held-to-Maturity, Estimated Fair Value	8,651
Obligations of U.S. Government Agencies
Schedule of Trading Securities and Other Trading Assets [Line Items]
Available-for-Sale, Amortized Cost	19,518
Available-for-sale Securities, Gross Unrealized Gains	213
Available-for-sale Securities, Gross Unrealized Losses	0
Available-for-sale securities, Estimated Fair Value	19,731
Tax-exempt and taxable obligations of states and municipal subdivisions
Schedule of Trading Securities and Other Trading Assets [Line Items]
Available-for-Sale, Amortized Cost	92,465
Available-for-sale Securities, Gross Unrealized Gains	2,103
Available-for-sale Securities, Gross Unrealized Losses	218
Available-for-sale securities, Estimated Fair Value	94,350
Held-to-Maturity, Amortized Cost	6,000
Held-to-maturity Securities, Gross Unrealized Gains	1,385
Held-to-maturity Securities, Gross Unrealized Losses	0
Held-to-Maturity, Estimated Fair Value	7,385
Mortgage-backed securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Available-for-Sale, Amortized Cost	97,771
Available-for-sale Securities, Gross Unrealized Gains	1,672
Available-for-sale Securities, Gross Unrealized Losses	59
Available-for-sale securities, Estimated Fair Value	99,384
Held-to-Maturity, Amortized Cost	1,233
Held-to-maturity Securities, Gross Unrealized Gains	33
Held-to-maturity Securities, Gross Unrealized Losses	0
Held-to-Maturity, Estimated Fair Value	1,266
Corporate obligations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Available-for-Sale, Amortized Cost	26,311
Available-for-sale Securities, Gross Unrealized Gains	136
Available-for-sale Securities, Gross Unrealized Losses	1,246
Available-for-sale securities, Estimated Fair Value	25,201
Other
Schedule of Trading Securities and Other Trading Assets [Line Items]
Available-for-Sale, Amortized Cost	1,255
Available-for-sale Securities, Gross Unrealized Gains	0
Available-for-sale Securities, Gross Unrealized Losses	290
Available-for-sale securities, Estimated Fair Value	$ 965

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

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Amount of unrealized gain before deducting unrealized loss on investments in debt and equity securities classified as available-for-sale securities.

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Amount of unrealized loss before deducting unrealized gain on investments in available-for-sale securities.

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

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