Investments - Investment Securities Continuous Unrealized Loss Position (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Investment [Line Items]
Fair Value of Investments with Unrealized Losses, Less Than 12 months	$ 168,622	$ 50,561
Gross Unrealized Loss, Less Than 12 Months	6,789	1,828
Fair Value of Investments with Unrealized Losses, 12 Months or Greater	14,363	2,345
Held-to-Maturity, Gross Unrealized Losses, Total	1,552	299
Gross Unrealized Losses, 12 Months or Greater	1,751	181
Fair Value of Investments with Unrealized Losses, Total	182,985	52,906
Gross Unrealized Losses, Total	8,540	2,009
Total available-for-sale securities [Member]
Investment [Line Items]
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months	112,527	27,797
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months	5,647	1,710
Fair Value of Investments with Unrealized Losses, 12 Months or Greater	5,102
Gross Unrealized Losses, 12 Months or Greater	1,341
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total	117,629	27,797
Available-for Sale, Gross Unrealized Losses, Total	6,988	1,710
Total available-for-sale securities [Member] | Corporate bonds [Member]
Investment [Line Items]
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months	18,101	6,411
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months	245	24
Fair Value of Investments with Unrealized Losses, 12 Months or Greater
Gross Unrealized Losses, 12 Months or Greater
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total	18,101	6,411
Available-for Sale, Gross Unrealized Losses, Total	245	24
Total available-for-sale securities [Member] | States and political subdivisions [Member]
Investment [Line Items]
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months	85,605	6,281
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months	4,375	142
Fair Value of Investments with Unrealized Losses, 12 Months or Greater
Gross Unrealized Losses, 12 Months or Greater
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total	85,605	6,281
Available-for Sale, Gross Unrealized Losses, Total	4,375	142
Total available-for-sale securities [Member] | U.S. agency-based mortgage-backed securities [Member]
Investment [Line Items]
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months	2,938	10,919
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months	700	859
Fair Value of Investments with Unrealized Losses, 12 Months or Greater	5,102
Gross Unrealized Losses, 12 Months or Greater	1,341
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total	8,040	10,919
Available-for Sale, Gross Unrealized Losses, Total	2,041	859
Total available-for-sale securities [Member] | Equity Securities [Member]
Investment [Line Items]
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months	5,883	4,186
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months	327	685
Fair Value of Investments with Unrealized Losses, 12 Months or Greater
Gross Unrealized Losses, 12 Months or Greater
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total	5,883	4,186
Available-for Sale, Gross Unrealized Losses, Total	327	685
Total held-to-maturity securities [Member]
Investment [Line Items]
Held-to-Maturity, Fair Value of Investments with Unrealized Losses, Less Than 12 Months	56,095	22,764
Held-to-Maturity, Gross Unrealized Losses, Less Than 12 Months	1,142	118
Held-to-Maturity, Fair Value of Investments with Unrealized Losses 12 Months or Greater	9,261	2,345
Held-to-Maturity, Gross Unrealized Losses, 12 Months or Greater	410	181
Held-to-Maturity, Fair Value of Investments with Unrealized Losses, Total	65,356	25,109
Held-to-Maturity, Gross Unrealized Losses, Total	1,552	299
Total held-to-maturity securities [Member] | Corporate bonds [Member]
Investment [Line Items]
Held-to-Maturity, Fair Value of Investments with Unrealized Losses, Less Than 12 Months	16,397	10,734
Held-to-Maturity, Gross Unrealized Losses, Less Than 12 Months	17	12
Held-to-Maturity, Fair Value of Investments with Unrealized Losses 12 Months or Greater	2,271
Held-to-Maturity, Gross Unrealized Losses, 12 Months or Greater	2
Held-to-Maturity, Fair Value of Investments with Unrealized Losses, Total	18,668	10,734
Held-to-Maturity, Gross Unrealized Losses, Total	19	12
Total held-to-maturity securities [Member] | States and political subdivisions [Member]
Investment [Line Items]
Held-to-Maturity, Fair Value of Investments with Unrealized Losses, Less Than 12 Months	39,698	11,913
Held-to-Maturity, Gross Unrealized Losses, Less Than 12 Months	1,125	106
Held-to-Maturity, Fair Value of Investments with Unrealized Losses 12 Months or Greater	5,028
Held-to-Maturity, Gross Unrealized Losses, 12 Months or Greater	237
Held-to-Maturity, Fair Value of Investments with Unrealized Losses, Total	44,726	11,913
Held-to-Maturity, Gross Unrealized Losses, Total	1,362	106
Total held-to-maturity securities [Member] | U.S. agency-based mortgage-backed securities [Member]
Investment [Line Items]
Held-to-Maturity, Fair Value of Investments with Unrealized Losses, Less Than 12 Months		117
Held-to-Maturity, Fair Value of Investments with Unrealized Losses 12 Months or Greater		68
Held-to-Maturity, Gross Unrealized Losses, 12 Months or Greater		5
Held-to-Maturity, Fair Value of Investments with Unrealized Losses, Total		185
Held-to-Maturity, Gross Unrealized Losses, Total		5
Total held-to-maturity securities [Member] | Asset-backed securities [Member]
Investment [Line Items]
Held-to-Maturity, Fair Value of Investments with Unrealized Losses, Less Than 12 Months
Held-to-Maturity, Gross Unrealized Losses, Less Than 12 Months
Held-to-Maturity, Fair Value of Investments with Unrealized Losses 12 Months or Greater	1,962	2,277
Held-to-Maturity, Gross Unrealized Losses, 12 Months or Greater	171	176
Held-to-Maturity, Fair Value of Investments with Unrealized Losses, Total	1,962	2,277
Held-to-Maturity, Gross Unrealized Losses, Total	$ 171	$ 176