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Investments - Investment Securities Continuous Unrealized Loss Position (Detail) - USD ($) $ in Thousands | Mar. 31, 2019 | Dec. 31, 2018
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Investment [Line Items] |  |
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Less Than 12 Months | $ 3,728 | $ 48,682
Held-to-maturity securities, Gross Unrealized Losses, Less Than 12 Months | 5 | 99
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, 12 Months or Greater | 159,421 | 278,163
Held-to-maturity securities, Gross Unrealized Losses, 12 Months or Greater | 594 | 2,867
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Total | 163,149 | 326,845
Held-to-maturity securities, Gross Unrealized Losses, Total | 599 | 2,966
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months | 7,691 | 75,208
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months | 8 | 360
Available-for Sale, Fair Value of Investments with Unrealized Losses, 12 Months or Greater | 172,166 | 218,832
Available-for Sale, Gross Unrealized Losses, 12 Months or Greater | 1,693 | 4,440
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total | 179,857 | 294,040
Available-for Sale, Gross Unrealized Losses, Total | 1,701 | 4,800
Fair Value of Investments with Unrealized Losses, Less Than 12 months | 11,419 | 123,890
Gross Unrealized Loss, Less Than 12 Months | 13 | 459
Fair Value of Investments with Unrealized Losses, 12 Months or Greater | 331,587 | 496,995
Gross Unrealized Losses, 12 Months or Greater | 2,287 | 7,307
Fair Value of Investments with Unrealized Losses, Total | 343,006 | 620,885
Gross Unrealized Losses, Total | 2,300 | 7,766
States and Political Subdivisions [Member] |  |
Investment [Line Items] |  |
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Less Than 12 Months |  | 28,369
Held-to-maturity securities, Gross Unrealized Losses, Less Than 12 Months |  | 59
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, 12 Months or Greater | 78,248 | 180,550
Held-to-maturity securities, Gross Unrealized Losses, 12 Months or Greater | 242 | 2,025
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Total | 78,248 | 208,919
Held-to-maturity securities, Gross Unrealized Losses, Total | 242 | 2,084
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months | 1,957 | 16,109
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months | 1 | 81
Available-for Sale, Fair Value of Investments with Unrealized Losses, 12 Months or Greater | 27,768 | 76,255
Available-for Sale, Gross Unrealized Losses, 12 Months or Greater | 394 | 2,037
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total | 29,725 | 92,364
Available-for Sale, Gross Unrealized Losses, Total | 395 | 2,118
Corporate Bonds [Member] |  |
Investment [Line Items] |  |
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Less Than 12 Months | 3,728 | 17,448
Held-to-maturity securities, Gross Unrealized Losses, Less Than 12 Months | 5 | 36
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, 12 Months or Greater | 37,828 | 48,315
Held-to-maturity securities, Gross Unrealized Losses, 12 Months or Greater | 150 | 419
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Total | 41,556 | 65,763
Held-to-maturity securities, Gross Unrealized Losses, Total | 155 | 455
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months | 5,734 | 59,099
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months | 7 | 279
Available-for Sale, Fair Value of Investments with Unrealized Losses, 12 Months or Greater | 75,651 | 70,306
Available-for Sale, Gross Unrealized Losses, 12 Months or Greater | 210 | 654
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total | 81,385 | 129,405
Available-for Sale, Gross Unrealized Losses, Total | 217 | 933
U.S. Agency-Based Mortgage-Backed Securities [Member] |  |
Investment [Line Items] |  |
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, 12 Months or Greater | 2,248 | 2,287
Held-to-maturity securities, Gross Unrealized Losses, 12 Months or Greater | 56 | 80
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Total | 2,248 | 2,287
Held-to-maturity securities, Gross Unrealized Losses, Total | 56 | 80
Available-for Sale, Fair Value of Investments with Unrealized Losses, 12 Months or Greater | 12,281 | 12,515
Available-for Sale, Gross Unrealized Losses, 12 Months or Greater | 180 | 320
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total | 12,281 | 12,515
Available-for Sale, Gross Unrealized Losses, Total | 180 | 320
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 |  | 2,865
Held-to-maturity securities, Gross Unrealized Losses, Less Than 12 Months |  | 4
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, 12 Months or Greater | 40,587 | 46,486
Held-to-maturity securities, Gross Unrealized Losses, 12 Months or Greater | 140 | 335
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Total | 40,587 | 49,351
Held-to-maturity securities, Gross Unrealized Losses, Total | 140 | 339
Available-for Sale, Fair Value of Investments with Unrealized Losses, 12 Months or Greater | 56,466 | 59,756
Available-for Sale, Gross Unrealized Losses, 12 Months or Greater | 909 | 1,429
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total | 56,466 | 59,756
Available-for Sale, Gross Unrealized Losses, Total | 909 | 1,429
Asset-Backed Securities [Member] |  |
Investment [Line Items] |  |
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, 12 Months or Greater | 510 | 525
Held-to-maturity securities, Gross Unrealized Losses, 12 Months or Greater | 6 | 8
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Total | 510 | 525
Held-to-maturity securities, Gross Unrealized Losses, Total | $ 6 | $ 8

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