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Investments - Investment Securities Continuous Unrealized Loss Position (Detail) - USD ($) $ in Thousands | Dec. 31, 2017 | Dec. 31, 2016
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Investment [Line Items] |  |
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Less Than 12 Months | $ 218,074 | $ 206,129
Held-to-maturity securities, Gross Unrealized Losses, Less Than 12 Months | 972 | 3,283
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, 12 Months or Greater | 28,931 | 7,465
Held-to-maturity securities, Gross Unrealized Losses, 12 Months or Greater | 403 | 122
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Total | 247,005 | 213,594
Held-to-maturity securities, Gross Unrealized Losses, Total | 1,375 | 3,405
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months | 135,576 | 173,573
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months | 520 | 4,113
Available-for Sale, Fair Value of Investments with Unrealized Losses, 12 Months or Greater | 76,085 | 16,052
Available-for Sale, Gross Unrealized Losses, 12 Months or Greater | 2,194 | 1,454
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total | 211,661 | 189,625
Available-for Sale, Gross Unrealized Losses, Total | 2,714 | 5,567
Fair Value of Investments with Unrealized Losses, Less Than 12 months | 353,650 | 379,702
Gross Unrealized Loss, Less Than 12 Months | 1,492 | 7,396
Fair Value of Investments with Unrealized Losses, 12 Months or Greater | 105,016 | 23,517
Gross Unrealized Losses, 12 Months or Greater | 2,597 | 1,576
Fair Value of Investments with Unrealized Losses, Total | 458,666 | 403,219
Gross Unrealized Losses, Total | 4,089 | 8,972
States and Political Subdivisions [Member] |  |
Investment [Line Items] |  |
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Less Than 12 Months | 110,698 | 157,507
Held-to-maturity securities, Gross Unrealized Losses, Less Than 12 Months | 654 | 3,014
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, 12 Months or Greater | 19,895 |
Held-to-maturity securities, Gross Unrealized Losses, 12 Months or Greater | 301 |
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Total | 130,593 | 157,507
Held-to-maturity securities, Gross Unrealized Losses, Total | 955 | 3,014
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months | 23,365 | 73,505
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months | 86 | 2,976
Available-for Sale, Fair Value of Investments with Unrealized Losses, 12 Months or Greater | 19,153 | 4,523
Available-for Sale, Gross Unrealized Losses, 12 Months or Greater | 491 | 239
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total | 42,518 | 78,028
Available-for Sale, Gross Unrealized Losses, Total | 577 | 3,215
Corporate Bonds [Member] |  |
Investment [Line Items] |  |
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Less Than 12 Months | 56,425 | 44,654
Held-to-maturity securities, Gross Unrealized Losses, Less Than 12 Months | 156 | 202
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, 12 Months or Greater | 4,121 | 6,292
Held-to-maturity securities, Gross Unrealized Losses, 12 Months or Greater | 11 | 63
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Total | 60,546 | 50,946
Held-to-maturity securities, Gross Unrealized Losses, Total | 167 | 265
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months | 82,795 | 41,419
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months | 171 | 111
Available-for Sale, Fair Value of Investments with Unrealized Losses, 12 Months or Greater | 5,888 | 7,922
Available-for Sale, Gross Unrealized Losses, 12 Months or Greater | 41 | 160
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total | 88,683 | 49,341
Available-for Sale, Gross Unrealized Losses, Total | 212 | 271
Asset-Backed Securities [Member] |  |
Investment [Line Items] |  |
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, 12 Months or Greater | 967 | 1,173
Held-to-maturity securities, Gross Unrealized Losses, 12 Months or Greater | 15 | 59
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Total | 967 | 1,173
Held-to-maturity securities, Gross Unrealized Losses, Total | 15 | 59
U.S. Agency-Based Mortgage-Backed Securities [Member] |  |
Investment [Line Items] |  |
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Less Than 12 Months | 2,798 |
Held-to-maturity securities, Gross Unrealized Losses, Less Than 12 Months | 40 |
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Total | 2,798 |
Held-to-maturity securities, Gross Unrealized Losses, Total | 40 |
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months | 14,686 | 3,702
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months | 59 | 48
Available-for Sale, Fair Value of Investments with Unrealized Losses, 12 Months or Greater | 3,328 | 3,607
Available-for Sale, Gross Unrealized Losses, 12 Months or Greater | 740 | 1,055
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total | 18,014 | 7,309
Available-for Sale, Gross Unrealized Losses, Total | 799 | 1,103
U.S. Treasury Securities and Obligations of U.S. Government Agencies [Member] |  |
Investment [Line Items] |  |
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Less Than 12 Months | 48,153 | 3,968
Held-to-maturity securities, Gross Unrealized Losses, Less Than 12 Months | 122 | 67
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, 12 Months or Greater | 3,948 |
Held-to-maturity securities, Gross Unrealized Losses, 12 Months or Greater | 76 |
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Total | 52,101 | 3,968
Held-to-maturity securities, Gross Unrealized Losses, Total | 198 | 67
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months | 14,730 | 54,947
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months | 204 | 978
Available-for Sale, Fair Value of Investments with Unrealized Losses, 12 Months or Greater | 47,716 |
Available-for Sale, Gross Unrealized Losses, 12 Months or Greater | 922 |
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total | 62,446 | 54,947
Available-for Sale, Gross Unrealized Losses, Total | $ 1,126 | $ 978

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