Fair Value Measurements - Schedule of Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Fair Value Measurements Disclosure [Line Items]
Securities available for sale/Available-for-sale, Fair Value	$ 10,271
Securities available for sale/Available-for-sale, Fair Value	225,232	155,905
Equity Securities [Member]
Fair Value Measurements Disclosure [Line Items]
Securities available for sale/Available-for-sale, Fair Value	12,260	6,766
Fair Value, Measurements, Recurring [Member]
Fair Value Measurements Disclosure [Line Items]
Securities available for sale/Available-for-sale, Fair Value	225,232	155,905
Fair Value, Measurements, Recurring [Member] | Other Investment [Member]
Fair Value Measurements Disclosure [Line Items]
Securities available for sale/Available-for-sale, Fair Value	10,271
Fair Value, Measurements, Recurring [Member] | Equity Securities [Member] | Common Stock
Fair Value Measurements Disclosure [Line Items]
Securities available for sale/Available-for-sale, Fair Value	12,260	6,766
Fair Value, Measurements, Recurring [Member] | Debt Securities [Member]
Fair Value Measurements Disclosure [Line Items]
Securities available for sale/Available-for-sale, Fair Value	202,701	149,139
Fair Value, Measurements, Recurring [Member] | Debt Securities [Member] | States and political subdivisions [Member]
Fair Value Measurements Disclosure [Line Items]
Securities available for sale/Available-for-sale, Fair Value	130,558	98,242
Fair Value, Measurements, Recurring [Member] | Debt Securities [Member] | Corporate bonds [Member]
Fair Value Measurements Disclosure [Line Items]
Securities available for sale/Available-for-sale, Fair Value	64,102	39,810
Fair Value, Measurements, Recurring [Member] | Debt Securities [Member] | U.S. agency-based mortgage-backed securities [Member]
Fair Value Measurements Disclosure [Line Items]
Securities available for sale/Available-for-sale, Fair Value	8,041	11,087
Fair Value, Inputs, Level 1 [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value Measurements Disclosure [Line Items]
Securities available for sale/Available-for-sale, Fair Value	12,260	6,766
Fair Value, Inputs, Level 1 [Member] | Fair Value, Measurements, Recurring [Member] | Other Investment [Member]
Fair Value Measurements Disclosure [Line Items]
Securities available for sale/Available-for-sale, Fair Value
Fair Value, Inputs, Level 1 [Member] | Fair Value, Measurements, Recurring [Member] | Equity Securities [Member] | Common Stock
Fair Value Measurements Disclosure [Line Items]
Securities available for sale/Available-for-sale, Fair Value	12,260	6,766
Fair Value, Inputs, Level 1 [Member] | Fair Value, Measurements, Recurring [Member] | Debt Securities [Member]
Fair Value Measurements Disclosure [Line Items]
Securities available for sale/Available-for-sale, Fair Value
Fair Value, Inputs, Level 1 [Member] | Fair Value, Measurements, Recurring [Member] | Debt Securities [Member] | States and political subdivisions [Member]
Fair Value Measurements Disclosure [Line Items]
Securities available for sale/Available-for-sale, Fair Value
Fair Value, Inputs, Level 1 [Member] | Fair Value, Measurements, Recurring [Member] | Debt Securities [Member] | Corporate bonds [Member]
Fair Value Measurements Disclosure [Line Items]
Securities available for sale/Available-for-sale, Fair Value
Fair Value, Inputs, Level 1 [Member] | Fair Value, Measurements, Recurring [Member] | Debt Securities [Member] | U.S. agency-based mortgage-backed securities [Member]
Fair Value Measurements Disclosure [Line Items]
Securities available for sale/Available-for-sale, Fair Value
Fair Value, Inputs, Level 2 [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value Measurements Disclosure [Line Items]
Securities available for sale/Available-for-sale, Fair Value	202,701	149,139
Fair Value, Inputs, Level 2 [Member] | Fair Value, Measurements, Recurring [Member] | Other Investment [Member]
Fair Value Measurements Disclosure [Line Items]
Securities available for sale/Available-for-sale, Fair Value
Fair Value, Inputs, Level 2 [Member] | Fair Value, Measurements, Recurring [Member] | Equity Securities [Member] | Common Stock
Fair Value Measurements Disclosure [Line Items]
Securities available for sale/Available-for-sale, Fair Value
Fair Value, Inputs, Level 2 [Member] | Fair Value, Measurements, Recurring [Member] | Debt Securities [Member]
Fair Value Measurements Disclosure [Line Items]
Securities available for sale/Available-for-sale, Fair Value	202,701	149,139
Fair Value, Inputs, Level 2 [Member] | Fair Value, Measurements, Recurring [Member] | Debt Securities [Member] | States and political subdivisions [Member]
Fair Value Measurements Disclosure [Line Items]
Securities available for sale/Available-for-sale, Fair Value	130,558	98,242
Fair Value, Inputs, Level 2 [Member] | Fair Value, Measurements, Recurring [Member] | Debt Securities [Member] | Corporate bonds [Member]
Fair Value Measurements Disclosure [Line Items]
Securities available for sale/Available-for-sale, Fair Value	64,102	39,810
Fair Value, Inputs, Level 2 [Member] | Fair Value, Measurements, Recurring [Member] | Debt Securities [Member] | U.S. agency-based mortgage-backed securities [Member]
Fair Value Measurements Disclosure [Line Items]
Securities available for sale/Available-for-sale, Fair Value	8,041	11,087
Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value Measurements Disclosure [Line Items]
Securities available for sale/Available-for-sale, Fair Value	10,271
Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Recurring [Member] | Other Investment [Member]
Fair Value Measurements Disclosure [Line Items]
Securities available for sale/Available-for-sale, Fair Value	10,271
Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Recurring [Member] | Equity Securities [Member] | Common Stock
Fair Value Measurements Disclosure [Line Items]
Securities available for sale/Available-for-sale, Fair Value
Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Recurring [Member] | Debt Securities [Member]
Fair Value Measurements Disclosure [Line Items]
Securities available for sale/Available-for-sale, Fair Value
Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Recurring [Member] | Debt Securities [Member] | States and political subdivisions [Member]
Fair Value Measurements Disclosure [Line Items]
Securities available for sale/Available-for-sale, Fair Value
Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Recurring [Member] | Debt Securities [Member] | Corporate bonds [Member]
Fair Value Measurements Disclosure [Line Items]
Securities available for sale/Available-for-sale, Fair Value
Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Recurring [Member] | Debt Securities [Member] | U.S. agency-based mortgage-backed securities [Member]
Fair Value Measurements Disclosure [Line Items]
Securities available for sale/Available-for-sale, Fair Value