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

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- Definition Amount of accumulated pre-tax unrealized loss on investments in available-for-sale securities that have been in a loss position for less than twelve months impacting investments. + References No definition available. + Details
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- Definition This item represents the excess of amortized cost basis over fair value of securities in a loss position and which are categorized as held-to-maturity. + References No definition available. + Details
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- Definition Amount of accumulated pre-tax unrealized loss on investments in held-to-maturity securities that have been in a loss position for twelve months or longer. + References No definition available. + Details
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