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Investments - Investment Securities Continuous Unrealized Loss Position (Detail) - USD ($) $ in Thousands | Jun. 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 | $ 337,721 | $ 218,074
Held-to-maturity securities, Gross Unrealized Losses, Less Than 12 Months | 3,813 | 972
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, 12 Months or Greater | 26,320 | 28,931
Held-to-maturity securities, Gross Unrealized Losses, 12 Months or Greater | 574 | 403
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Total | 364,041 | 247,005
Held-to-maturity securities, Gross Unrealized Losses, Total | 4,387 | 1,375
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months | 228,835 | 135,576
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months | 3,314 | 520
Available-for Sale, Fair Value of Investments with Unrealized Losses, 12 Months or Greater | 65,372 | 76,085
Available-for Sale, Gross Unrealized Losses, 12 Months or Greater | 2,710 | 2,194
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total | 294,207 | 211,661
Available-for Sale, Gross Unrealized Losses, Total | 6,024 | 2,714
Fair Value of Investments with Unrealized Losses, Less Than 12 months | 566,556 | 353,650
Gross Unrealized Loss, Less Than 12 Months | 7,127 | 1,492
Fair Value of Investments with Unrealized Losses, 12 Months or Greater | 91,692 | 105,016
Gross Unrealized Losses, 12 Months or Greater | 3,284 | 2,597
Fair Value of Investments with Unrealized Losses, Total | 658,248 | 458,666
Gross Unrealized Losses, Total | 10,411 | 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 | 204,459 | 110,698
Held-to-maturity securities, Gross Unrealized Losses, Less Than 12 Months | 2,568 | 654
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, 12 Months or Greater | 12,424 | 19,895
Held-to-maturity securities, Gross Unrealized Losses, 12 Months or Greater | 323 | 301
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Total | 216,883 | 130,593
Held-to-maturity securities, Gross Unrealized Losses, Total | 2,891 | 955
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months | 77,359 | 23,365
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months | 1,356 | 86
Available-for Sale, Fair Value of Investments with Unrealized Losses, 12 Months or Greater | 15,476 | 19,153
Available-for Sale, Gross Unrealized Losses, 12 Months or Greater | 1,139 | 491
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total | 92,835 | 42,518
Available-for Sale, Gross Unrealized Losses, Total | 2,495 | 577
Corporate Bonds [Member] |  |
Investment [Line Items] |  |
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Less Than 12 Months | 86,816 | 56,425
Held-to-maturity securities, Gross Unrealized Losses, Less Than 12 Months | 703 | 156
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, 12 Months or Greater | 2,237 | 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 | 89,053 | 60,546
Held-to-maturity securities, Gross Unrealized Losses, Total | 724 | 167
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months | 121,848 | 82,795
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months | 1,016 | 171
Available-for Sale, Fair Value of Investments with Unrealized Losses, 12 Months or Greater | 1,021 | 5,888
Available-for Sale, Gross Unrealized Losses, 12 Months or Greater | 35 | 41
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total | 122,869 | 88,683
Available-for Sale, Gross Unrealized Losses, Total | 1,051 | 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 | 2,524 | 2,798
Held-to-maturity securities, Gross Unrealized Losses, Less Than 12 Months | 71 | 40
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Total | 2,524 | 2,798
Held-to-maturity securities, Gross Unrealized Losses, Total | 71 | 40
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months | 11,557 | 14,686
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months | 312 | 59
Available-for Sale, Fair Value of Investments with Unrealized Losses, 12 Months or Greater | 1,805 | 3,328
Available-for Sale, Gross Unrealized Losses, 12 Months or Greater | 56 | 740
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total | 13,362 | 18,014
Available-for Sale, Gross Unrealized Losses, Total | 368 | 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 | 43,357 | 48,153
Held-to-maturity securities, Gross Unrealized Losses, Less Than 12 Months | 461 | 122
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, 12 Months or Greater | 11,659 | 3,948
Held-to-maturity securities, Gross Unrealized Losses, 12 Months or Greater | 230 | 76
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Total | 55,016 | 52,101
Held-to-maturity securities, Gross Unrealized Losses, Total | 691 | 198
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months | 18,071 | 14,730
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months | 630 | 204
Available-for Sale, Fair Value of Investments with Unrealized Losses, 12 Months or Greater | 47,070 | 47,716
Available-for Sale, Gross Unrealized Losses, 12 Months or Greater | 1,480 | 922
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total | 65,141 | 62,446
Available-for Sale, Gross Unrealized Losses, Total | 2,110 | 1,126
Asset-Backed Securities [Member] |  |
Investment [Line Items] |  |
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Less Than 12 Months | 565 |
Held-to-maturity securities, Gross Unrealized Losses, Less Than 12 Months | 10 |
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, 12 Months or Greater |  | 967
Held-to-maturity securities, Gross Unrealized Losses, 12 Months or Greater |  | 15
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Total | 565 | 967
Held-to-maturity securities, Gross Unrealized Losses, Total | $ 10 | $ 15

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