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Investments - Investment Securities Continuous Unrealized Loss Position (Detail) - USD ($) $ in Thousands | Mar. 31, 2018 | Dec. 31, 2017
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Investment [Line Items] |  |
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Less Than 12 Months | $ 353,557 | $ 218,074
Held-to-maturity securities, Gross Unrealized Losses, Less Than 12 Months | 3,955 | 972
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, 12 Months or Greater | 21,287 | 28,931
Held-to-maturity securities, Gross Unrealized Losses, 12 Months or Greater | 564 | 403
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Total | 374,844 | 247,005
Held-to-maturity securities, Gross Unrealized Losses, Total | 4,519 | 1,375
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months | 267,995 | 135,576
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months | 3,727 | 520
Available-for Sale, Fair Value of Investments with Unrealized Losses, 12 Months or Greater | 51,785 | 76,085
Available-for Sale, Gross Unrealized Losses, 12 Months or Greater | 2,777 | 2,194
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total | 319,780 | 211,661
Available-for Sale, Gross Unrealized Losses, Total | 6,504 | 2,714
Fair Value of Investments with Unrealized Losses, Less Than 12 months | 621,552 | 353,650
Gross Unrealized Loss, Less Than 12 Months | 7,682 | 1,492
Fair Value of Investments with Unrealized Losses, 12 Months or Greater | 73,072 | 105,016
Gross Unrealized Losses, 12 Months or Greater | 3,341 | 2,597
Fair Value of Investments with Unrealized Losses, Total | 694,624 | 458,666
Gross Unrealized Losses, Total | 11,023 | 4,089
States and Political Subdivisions [Member] |  |
Investment [Line Items] |  |
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Less Than 12 Months | 212,693 | 110,698
Held-to-maturity securities, Gross Unrealized Losses, Less Than 12 Months | 2,810 | 654
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, 12 Months or Greater | 14,513 | 19,895
Held-to-maturity securities, Gross Unrealized Losses, 12 Months or Greater | 438 | 301
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Total | 227,206 | 130,593
Held-to-maturity securities, Gross Unrealized Losses, Total | 3,248 | 955
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months | 89,896 | 23,365
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months | 1,620 | 86
Available-for Sale, Fair Value of Investments with Unrealized Losses, 12 Months or Greater | 8,892 | 19,153
Available-for Sale, Gross Unrealized Losses, 12 Months or Greater | 790 | 491
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total | 98,788 | 42,518
Available-for Sale, Gross Unrealized Losses, Total | 2,410 | 577
Corporate Bonds [Member] |  |
Investment [Line Items] |  |
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Less Than 12 Months | 89,227 | 56,425
Held-to-maturity securities, Gross Unrealized Losses, Less Than 12 Months | 629 | 156
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, 12 Months or Greater | 2,259 | 4,121
Held-to-maturity securities, Gross Unrealized Losses, 12 Months or Greater | 21 | 11
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Total | 91,486 | 60,546
Held-to-maturity securities, Gross Unrealized Losses, Total | 650 | 167
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months | 138,600 | 82,795
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months | 943 | 171
Available-for Sale, Fair Value of Investments with Unrealized Losses, 12 Months or Greater |  | 5,888
Available-for Sale, Gross Unrealized Losses, 12 Months or Greater |  | 41
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total | 138,600 | 88,683
Available-for Sale, Gross Unrealized Losses, Total | 943 | 212
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 | 2,613 | 2,798
Held-to-maturity securities, Gross Unrealized Losses, Less Than 12 Months | 72 | 40
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Total | 2,613 | 2,798
Held-to-maturity securities, Gross Unrealized Losses, Total | 72 | 40
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months | 14,477 | 14,686
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months | 451 | 59
Available-for Sale, Fair Value of Investments with Unrealized Losses, 12 Months or Greater | 2,508 | 3,328
Available-for Sale, Gross Unrealized Losses, 12 Months or Greater | 824 | 740
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total | 16,985 | 18,014
Available-for Sale, Gross Unrealized Losses, Total | 1,275 | 799
Asset-Backed Securities [Member] |  |
Investment [Line Items] |  |
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Less Than 12 Months | 535 |
Held-to-maturity securities, Gross Unrealized Losses, Less Than 12 Months | 5 |
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, 12 Months or Greater | 47 | 967
Held-to-maturity securities, Gross Unrealized Losses, 12 Months or Greater | 2 | 15
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Total | 582 | 967
Held-to-maturity securities, Gross Unrealized Losses, Total | 7 | 15
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 | 48,489 | 48,153
Held-to-maturity securities, Gross Unrealized Losses, Less Than 12 Months | 439 | 122
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, 12 Months or Greater | 4,468 | 3,948
Held-to-maturity securities, Gross Unrealized Losses, 12 Months or Greater | 103 | 76
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Total | 52,957 | 52,101
Held-to-maturity securities, Gross Unrealized Losses, Total | 542 | 198
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months | 25,022 | 14,730
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months | 713 | 204
Available-for Sale, Fair Value of Investments with Unrealized Losses, 12 Months or Greater | 40,385 | 47,716
Available-for Sale, Gross Unrealized Losses, 12 Months or Greater | 1,163 | 922
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total | 65,407 | 62,446
Available-for Sale, Gross Unrealized Losses, Total | $ 1,876 | $ 1,126

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