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Investments - Investment Securities Continuous Unrealized Loss Position (Detail) - USD ($) $ in Thousands | Dec. 31, 2015 | Dec. 31, 2014
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Investment [Line Items] |  |
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value | $ 165,286 | $ 150,147
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less Than Twelve Months, Accumulated Loss | 751 | 582
Held-to-maturity Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value | 19,556 | 1,662
Held-to-maturity Securities, Continuous Unrealized Loss Position Twelve Months Or Longer, Accumulated Loss | 200 | 82
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value | 184,842 | 151,809
Held-to-maturity Securities, Gross Unrealized Losses | 951 | 664
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months | 148,851 | 110,622
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months | 1,521 | 487
Available-for Sale, Fair Value of Investments with Unrealized Losses, 12 Months or Greater | 15,449 | 18,104
Available-for Sale, Gross Unrealized Losses, 12 Months or Greater | 1,765 | 3,007
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total | 164,300 | 128,726
Available-for Sale, Gross Unrealized Losses, Total | 3,286 | 3,494
Fair Value of Investments with Unrealized Losses, Less Than 12 months | 314,137 | 260,769
Gross Unrealized Loss, Less Than 12 Months | 2,272 | 1,069
Fair Value of Investments with Unrealized Losses, 12 Months or Greater | 35,005 | 19,766
Gross Unrealized Losses, 12 Months or Greater | 1,965 | 3,089
Fair Value of Investments with Unrealized Losses, Total | 349,142 | 280,535
Gross Unrealized Losses, Total | 4,237 | 4,158
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 | 2,980 | 3,385
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less Than Twelve Months, Accumulated Loss | 4 | 2
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value | 2,980 | 3,385
Held-to-maturity Securities, Gross Unrealized Losses | 4 | 2
Corporate Bonds [Member] |  |
Investment [Line Items] |  |
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value | 128,436 | 129,788
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less Than Twelve Months, Accumulated Loss | 687 | 520
Held-to-maturity Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value | 18,139 |
Held-to-maturity Securities, Continuous Unrealized Loss Position Twelve Months Or Longer, Accumulated Loss | 123 |
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value | 146,575 | 129,788
Held-to-maturity Securities, Gross Unrealized Losses | 810 | 520
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months | 141,857 | 106,185
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months | 1,475 | 470
Available-for Sale, Fair Value of Investments with Unrealized Losses, 12 Months or Greater | 4,216 |
Available-for Sale, Gross Unrealized Losses, 12 Months or Greater | 11 |
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total | 146,073 | 106,185
Available-for Sale, Gross Unrealized Losses, Total | 1,486 | 470
States and Political Subdivisions [Member] |  |
Investment [Line Items] |  |
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value | 24,068 | 16,896
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less Than Twelve Months, Accumulated Loss | 45 | 58
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value | 24,068 | 16,896
Held-to-maturity Securities, Gross Unrealized Losses | 45 | 58
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months | 6,560 | 3,810
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months | 9 | 6
Available-for Sale, Fair Value of Investments with Unrealized Losses, 12 Months or Greater | 4,439 | 10,347
Available-for Sale, Gross Unrealized Losses, 12 Months or Greater | 198 | 1,666
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total | 10,999 | 14,157
Available-for Sale, Gross Unrealized Losses, Total | 207 | 1,672
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 | 78
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less Than Twelve Months, Accumulated Loss |  | 2
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 | 78
Held-to-maturity Securities, Gross Unrealized Losses | 1 | 2
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months | 434 | 627
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months | 37 | 11
Available-for Sale, Fair Value of Investments with Unrealized Losses, 12 Months or Greater | 6,794 | 7,757
Available-for Sale, Gross Unrealized Losses, 12 Months or Greater | 1,556 | 1,341
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total | 7,228 | 8,384
Available-for Sale, Gross Unrealized Losses, Total | 1,593 | 1,352
Asset-Backed Securities [Member] |  |
Investment [Line Items] |  |
Held-to-maturity Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value | 1,389 | 1,662
Held-to-maturity Securities, Continuous Unrealized Loss Position Twelve Months Or Longer, Accumulated Loss | 76 | 82
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value | 1,389 | 1,662
Held-to-maturity Securities, Gross Unrealized Losses | 76 | $ 82
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 |

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