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Investments - Investment Securities Continuous Unrealized Loss Position (Detail) (USD $) In Thousands, unless otherwise specified | Sep. 30, 2013 | Dec. 31, 2012
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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

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