Fair Value Measurements - Schedule of Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Fair Value Measurements Disclosure [Line Items]
Securities available for sale/Available-for-sale, Fair value	$ 332,104	$ 257,770
Equity securities [Member]
Fair Value Measurements Disclosure [Line Items]
Securities available for sale/Available-for-sale, Fair value	3,598	9,302
Other investments [Member]
Fair Value Measurements Disclosure [Line Items]
Securities available for sale/Available-for-sale, Fair value	11,228	10,591
Fair Value, Measurements, Recurring [Member]
Fair Value Measurements Disclosure [Line Items]
Securities available for sale/Available-for-sale, Fair value	332,104	257,770
Fair Value, Measurements, Recurring [Member] \| Debt Securities [Member]
Fair Value Measurements Disclosure [Line Items]
Securities available for sale/Available-for-sale, Fair value	317,278	237,877
Fair Value, Measurements, Recurring [Member] \| Other investments [Member]
Fair Value Measurements Disclosure [Line Items]
Securities available for sale/Available-for-sale, Fair value	11,228	10,591
Fair Value, Measurements, Recurring [Member] \| Common Stock [Member] \| Equity securities [Member]
Fair Value Measurements Disclosure [Line Items]
Securities available for sale/Available-for-sale, Fair value	3,598	9,302
Fair Value, Measurements, Recurring [Member] \| States and Political Subdivisions [Member] \| Debt Securities [Member]
Fair Value Measurements Disclosure [Line Items]
Securities available for sale/Available-for-sale, Fair value	156,789	150,375
Fair Value, Measurements, Recurring [Member] \| Corporate Bonds [Member] \| Debt Securities [Member]
Fair Value Measurements Disclosure [Line Items]
Securities available for sale/Available-for-sale, Fair value	152,670	80,428
Fair Value, Measurements, Recurring [Member] \| U.S. Agency-Based Mortgage-Backed Securities [Member] \| Debt Securities [Member]
Fair Value Measurements Disclosure [Line Items]
Securities available for sale/Available-for-sale, Fair value	7,819	7,074
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	3,598	9,302
Fair Value, Inputs, Level 1 [Member] \| Fair Value, Measurements, Recurring [Member] \| Common Stock [Member] \| Equity securities [Member]
Fair Value Measurements Disclosure [Line Items]
Securities available for sale/Available-for-sale, Fair value	3,598	9,302
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	317,278	237,877
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	317,278	237,877
Fair Value, Inputs, Level 2 [Member] \| Fair Value, Measurements, Recurring [Member] \| States and Political Subdivisions [Member] \| Debt Securities [Member]
Fair Value Measurements Disclosure [Line Items]
Securities available for sale/Available-for-sale, Fair value	156,789	150,375
Fair Value, Inputs, Level 2 [Member] \| Fair Value, Measurements, Recurring [Member] \| Corporate Bonds [Member] \| Debt Securities [Member]
Fair Value Measurements Disclosure [Line Items]
Securities available for sale/Available-for-sale, Fair value	152,670	80,428
Fair Value, Inputs, Level 2 [Member] \| Fair Value, Measurements, Recurring [Member] \| U.S. Agency-Based Mortgage-Backed Securities [Member] \| Debt Securities [Member]
Fair Value Measurements Disclosure [Line Items]
Securities available for sale/Available-for-sale, Fair value	7,819	7,074
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	11,228	10,591
Fair Value, Inputs, Level 3 [Member] \| Fair Value, Measurements, Recurring [Member] \| Other investments [Member]
Fair Value Measurements Disclosure [Line Items]
Securities available for sale/Available-for-sale, Fair value	$ 11,228	$ 10,591