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Investments - Investment Securities Continuous Unrealized Loss Position (Detail) - USD ($) $ in Thousands | Sep. 30, 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 | $ 338,886 | $ 218,074
Held-to-maturity securities, Gross Unrealized Losses, Less Than 12 Months | 4,837 | 972
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, 12 Months or Greater | 86,047 | 28,931
Held-to-maturity securities, Gross Unrealized Losses, 12 Months or Greater | 1,878 | 403
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Total | 424,933 | 247,005
Held-to-maturity securities, Gross Unrealized Losses, Total | 6,715 | 1,375
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months | 248,415 | 135,576
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months | 3,474 | 520
Available-for Sale, Fair Value of Investments with Unrealized Losses, 12 Months or Greater | 91,515 | 76,085
Available-for Sale, Gross Unrealized Losses, 12 Months or Greater | 4,199 | 2,194
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total | 339,930 | 211,661
Available-for Sale, Gross Unrealized Losses, Total | 7,673 | 2,714
Fair Value of Investments with Unrealized Losses, Less Than 12 months | 587,301 | 353,650
Gross Unrealized Loss, Less Than 12 Months | 8,311 | 1,492
Fair Value of Investments with Unrealized Losses, 12 Months or Greater | 177,562 | 105,016
Gross Unrealized Losses, 12 Months or Greater | 6,077 | 2,597
Fair Value of Investments with Unrealized Losses, Total | 764,863 | 458,666
Gross Unrealized Losses, Total | 14,388 | 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 | 237,284 | 110,698
Held-to-maturity securities, Gross Unrealized Losses, Less Than 12 Months | 3,737 | 654
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, 12 Months or Greater | 35,373 | 19,895
Held-to-maturity securities, Gross Unrealized Losses, 12 Months or Greater | 1,036 | 301
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Total | 272,657 | 130,593
Held-to-maturity securities, Gross Unrealized Losses, Total | 4,773 | 955
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months | 97,971 | 23,365
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months | 2,118 | 86
Available-for Sale, Fair Value of Investments with Unrealized Losses, 12 Months or Greater | 21,048 | 19,153
Available-for Sale, Gross Unrealized Losses, 12 Months or Greater | 1,736 | 491
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total | 119,019 | 42,518
Available-for Sale, Gross Unrealized Losses, Total | 3,854 | 577
Corporate Bonds [Member] |  |
Investment [Line Items] |  |
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Less Than 12 Months | 67,454 | 56,425
Held-to-maturity securities, Gross Unrealized Losses, Less Than 12 Months | 584 | 156
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, 12 Months or Greater | 21,100 | 4,121
Held-to-maturity securities, Gross Unrealized Losses, 12 Months or Greater | 295 | 11
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Total | 88,554 | 60,546
Held-to-maturity securities, Gross Unrealized Losses, Total | 879 | 167
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months | 133,712 | 82,795
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months | 772 | 171
Available-for Sale, Fair Value of Investments with Unrealized Losses, 12 Months or Greater | 9,615 | 5,888
Available-for Sale, Gross Unrealized Losses, 12 Months or Greater | 157 | 41
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total | 143,327 | 88,683
Available-for Sale, Gross Unrealized Losses, Total | 929 | 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 | 1,962 | 2,798
Held-to-maturity securities, Gross Unrealized Losses, Less Than 12 Months | 65 | 40
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, 12 Months or Greater | 482 |
Held-to-maturity securities, Gross Unrealized Losses, 12 Months or Greater | 22 |
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Total | 2,444 | 2,798
Held-to-maturity securities, Gross Unrealized Losses, Total | 87 | 40
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months | 8,356 | 14,686
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months | 288 | 59
Available-for Sale, Fair Value of Investments with Unrealized Losses, 12 Months or Greater | 4,424 | 3,328
Available-for Sale, Gross Unrealized Losses, 12 Months or Greater | 188 | 740
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total | 12,780 | 18,014
Available-for Sale, Gross Unrealized Losses, Total | 476 | 799
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 | 31,676 | 48,153
Held-to-maturity securities, Gross Unrealized Losses, Less Than 12 Months | 443 | 122
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, 12 Months or Greater | 29,054 | 3,948
Held-to-maturity securities, Gross Unrealized Losses, 12 Months or Greater | 523 | 76
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Total | 60,730 | 52,101
Held-to-maturity securities, Gross Unrealized Losses, Total | 966 | 198
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months | 8,376 | 14,730
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months | 296 | 204
Available-for Sale, Fair Value of Investments with Unrealized Losses, 12 Months or Greater | 56,428 | 47,716
Available-for Sale, Gross Unrealized Losses, 12 Months or Greater | 2,118 | 922
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total | 64,804 | 62,446
Available-for Sale, Gross Unrealized Losses, Total | 2,414 | 1,126
Asset-Backed Securities [Member] |  |
Investment [Line Items] |  |
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Less Than 12 Months | 510 |
Held-to-maturity securities, Gross Unrealized Losses, Less Than 12 Months | 8 |
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, 12 Months or Greater | 38 | 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 | 548 | 967
Held-to-maturity securities, Gross Unrealized Losses, Total | $ 10 | $ 15

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