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Investments - Investment Securities Continuous Unrealized Loss Position (Detail) - USD ($) $ in Thousands | Sep. 30, 2016 | Dec. 31, 2015
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Investment [Line Items] |  |
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value | $ 33,408 | $ 165,286
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less Than Twelve Months, Accumulated Loss | 152 | 751
Held-to-maturity Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value | 4,394 | 19,556
Held-to-maturity Securities, Continuous Unrealized Loss Position Twelve Months Or Longer, Accumulated Loss | 91 | 200
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value | 37,802 | 184,842
Held-to-maturity Securities, Gross Unrealized Losses | 243 | 951
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months | 33,086 | 148,851
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months | 425 | 1,521
Available-for Sale, Fair Value of Investments with Unrealized Losses, 12 Months or Greater | 19,576 | 15,449
Available-for Sale, Gross Unrealized Losses, 12 Months or Greater | 1,033 | 1,765
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total | 52,662 | 164,300
Available-for Sale, Gross Unrealized Losses, Total | 1,458 | 3,286
Fair Value of Investments with Unrealized Losses, Less Than 12 months | 66,494 | 314,137
Gross Unrealized Loss, Less Than 12 Months | 577 | 2,272
Fair Value of Investments with Unrealized Losses, 12 Months or Greater | 23,970 | 35,005
Gross Unrealized Losses, 12 Months or Greater | 1,124 | 1,965
Fair Value of Investments with Unrealized Losses, Total | 90,464 | 349,142
Gross Unrealized Losses, Total | 1,701 | 4,237
States and Political Subdivisions [Member] |  |
Investment [Line Items] |  |
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value | 23,075 | 24,068
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less Than Twelve Months, Accumulated Loss | 96 | 45
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value | 23,075 | 24,068
Held-to-maturity Securities, Gross Unrealized Losses | 96 | 45
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months | 22,147 | 6,560
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months | 328 | 9
Available-for Sale, Fair Value of Investments with Unrealized Losses, 12 Months or Greater | 4,746 | 4,439
Available-for Sale, Gross Unrealized Losses, 12 Months or Greater | 31 | 198
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total | 26,893 | 10,999
Available-for Sale, Gross Unrealized Losses, Total | 359 | 207
Corporate Bonds [Member] |  |
Investment [Line Items] |  |
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value | 9,696 | 128,436
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less Than Twelve Months, Accumulated Loss | 55 | 687
Held-to-maturity Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value | 3,173 | 18,139
Held-to-maturity Securities, Continuous Unrealized Loss Position Twelve Months Or Longer, Accumulated Loss | 26 | 123
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value | 12,869 | 146,575
Held-to-maturity Securities, Gross Unrealized Losses | 81 | 810
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months | 7,673 | 141,857
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months | 9 | 1,475
Available-for Sale, Fair Value of Investments with Unrealized Losses, 12 Months or Greater | 7,900 | 4,216
Available-for Sale, Gross Unrealized Losses, 12 Months or Greater | 205 | 11
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total | 15,573 | 146,073
Available-for Sale, Gross Unrealized Losses, Total | 214 | 1,486
Commercial Mortgage-Backed Securities [Member] |  |
Investment [Line Items] |  |
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value |  | 9,784
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less Than Twelve Months, Accumulated Loss |  | 15
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value |  | 9,784
Held-to-maturity Securities, Gross Unrealized Losses |  | 15
U.S. Agency-Based Mortgage-Backed Securities [Member] |  |
Investment [Line Items] |  |
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value |  | 18
Held-to-maturity Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value |  | 28
Held-to-maturity Securities, Continuous Unrealized Loss Position Twelve Months Or Longer, Accumulated Loss |  | 1
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value |  | 46
Held-to-maturity Securities, Gross Unrealized Losses |  | 1
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months |  | 434
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months |  | 37
Available-for Sale, Fair Value of Investments with Unrealized Losses, 12 Months or Greater | 6,930 | 6,794
Available-for Sale, Gross Unrealized Losses, 12 Months or Greater | 797 | 1,556
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total | 6,930 | 7,228
Available-for Sale, Gross Unrealized Losses, Total | 797 | 1,593
Asset-Backed Securities [Member] |  |
Investment [Line Items] |  |
Held-to-maturity Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value | 1,221 | 1,389
Held-to-maturity Securities, Continuous Unrealized Loss Position Twelve Months Or Longer, Accumulated Loss | 65 | 76
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value | 1,221 | 1,389
Held-to-maturity Securities, Gross Unrealized Losses | 65 | 76
U.S. Treasury Securities and Obligations of U.S. Government Agencies [Member] |  |
Investment [Line Items] |  |
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value | 637 | 2,980
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less Than Twelve Months, Accumulated Loss | 1 | 4
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value | 637 | 2,980
Held-to-maturity Securities, Gross Unrealized Losses | 1 | $ 4
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months | 3,266 |
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months | 88 |
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total | 3,266 |
Available-for Sale, Gross Unrealized Losses, Total | $ 88 |

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