Investments - Investment Securities Continuous Unrealized Loss Position (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Investment [Line Items]
Fair Value of Investments with Unrealized Losses, Less Than 12 months	$ 184,506	$ 50,561
Gross Unrealized Loss, Less Than 12 Months	6,885	1,828
Held-to-Maturity, Gross Unrealized Losses, Total	1,516	299
Fair Value of Investments with Unrealized Losses, 12 Months or Greater	2,056	2,345
Gross Unrealized Losses, 12 Months or Greater	180	181
Fair Value of Investments with Unrealized Losses, Total	186,562	52,906
Gross Unrealized Losses, Total	7,065	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	108,248	27,797
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months	5,549	1,710
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	108,248	27,797
Available-for Sale, Gross Unrealized Losses, Total	5,549	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	26,823	6,411
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months	273	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	26,823	6,411
Available-for Sale, Gross Unrealized Losses, Total	273	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	69,199	6,281
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months	2,567	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	69,199	6,281
Available-for Sale, Gross Unrealized Losses, Total	2,567	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	8,309	10,919
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months	1,981	859
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	8,309	10,919
Available-for Sale, Gross Unrealized Losses, Total	1,981	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	3,917	4,186
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months	728	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	3,917	4,186
Available-for Sale, Gross Unrealized Losses, Total	728	685
Total held-to-maturity securities [Member]
Investment [Line Items]
Held-to-Maturity, Fair Value of Investments with Unrealized Losses, Less Than 12 Months	76,258	22,764
Held-to-Maturity, Gross Unrealized Losses, Less Than 12 Months	1,336	118
Held-to-Maturity, Fair Value of Investments with Unrealized Losses 12 Months or Greater	2,056	2,345
Held-to-Maturity, Gross Unrealized Losses, 12 Months or Greater	180	181
Held-to-Maturity, Fair Value of Investments with Unrealized Losses, Total	78,314	25,109
Held-to-Maturity, Gross Unrealized Losses, Total	1,516	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	29,035	10,734
Held-to-Maturity, Gross Unrealized Losses, Less Than 12 Months	74	12
Held-to-Maturity, Fair Value of Investments with Unrealized Losses 12 Months or Greater
Held-to-Maturity, Gross Unrealized Losses, 12 Months or Greater
Held-to-Maturity, Fair Value of Investments with Unrealized Losses, Total	29,035	10,734
Held-to-Maturity, Gross Unrealized Losses, Total	74	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	47,223	11,913
Held-to-Maturity, Gross Unrealized Losses, Less Than 12 Months	1,262	106
Held-to-Maturity, Fair Value of Investments with Unrealized Losses 12 Months or Greater
Held-to-Maturity, Gross Unrealized Losses, 12 Months or Greater
Held-to-Maturity, Fair Value of Investments with Unrealized Losses, Total	47,223	11,913
Held-to-Maturity, Gross Unrealized Losses, Total	1,262	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, Gross Unrealized Losses, Less Than 12 Months
Held-to-Maturity, Fair Value of Investments with Unrealized Losses 12 Months or Greater	9	68
Held-to-Maturity, Gross Unrealized Losses, 12 Months or Greater		5
Held-to-Maturity, Fair Value of Investments with Unrealized Losses, Total	9	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	2,047	2,277
Held-to-Maturity, Gross Unrealized Losses, 12 Months or Greater	180	176
Held-to-Maturity, Fair Value of Investments with Unrealized Losses, Total	2,047	2,277
Held-to-Maturity, Gross Unrealized Losses, Total	$ 180	$ 176