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Investments - Investment Securities Continuous Unrealized Loss Position (Detail) - USD ($) $ in Thousands | Dec. 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 | $ 48,682 | $ 218,074
Held-to-maturity securities, Gross Unrealized Losses, Less Than 12 Months | 99 | 972
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, 12 Months or Greater | 278,163 | 28,931
Held-to-maturity securities, Gross Unrealized Losses, 12 Months or Greater | 2,867 | 403
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Total | 326,845 | 247,005
Held-to-maturity securities, Gross Unrealized Losses, Total | 2,966 | 1,375
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months | 75,208 | 135,576
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months | 360 | 520
Available-for Sale, Fair Value of Investments with Unrealized Losses, 12 Months or Greater | 218,832 | 76,085
Available-for Sale, Gross Unrealized Losses, 12 Months or Greater | 4,440 | 2,194
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total | 294,040 | 211,661
Available-for Sale, Gross Unrealized Losses, Total | 4,800 | 2,714
Fair Value of Investments with Unrealized Losses, Less Than 12 months | 123,890 | 353,650
Gross Unrealized Loss, Less Than 12 Months | 459 | 1,492
Fair Value of Investments with Unrealized Losses, 12 Months or Greater | 496,995 | 105,016
Gross Unrealized Losses, 12 Months or Greater | 7,307 | 2,597
Fair Value of Investments with Unrealized Losses, Total | 620,885 | 458,666
Gross Unrealized Losses, Total | 7,766 | 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 | 28,369 | 110,698
Held-to-maturity securities, Gross Unrealized Losses, Less Than 12 Months | 59 | 654
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, 12 Months or Greater | 180,550 | 19,895
Held-to-maturity securities, Gross Unrealized Losses, 12 Months or Greater | 2,025 | 301
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Total | 208,919 | 130,593
Held-to-maturity securities, Gross Unrealized Losses, Total | 2,084 | 955
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months | 16,109 | 23,365
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months | 81 | 86
Available-for Sale, Fair Value of Investments with Unrealized Losses, 12 Months or Greater | 76,255 | 19,153
Available-for Sale, Gross Unrealized Losses, 12 Months or Greater | 2,037 | 491
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total | 92,364 | 42,518
Available-for Sale, Gross Unrealized Losses, Total | 2,118 | 577
Corporate Bonds [Member] |  |
Investment [Line Items] |  |
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Less Than 12 Months | 17,448 | 56,425
Held-to-maturity securities, Gross Unrealized Losses, Less Than 12 Months | 36 | 156
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, 12 Months or Greater | 48,315 | 4,121
Held-to-maturity securities, Gross Unrealized Losses, 12 Months or Greater | 419 | 11
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Total | 65,763 | 60,546
Held-to-maturity securities, Gross Unrealized Losses, Total | 455 | 167
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months | 59,099 | 82,795
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months | 279 | 171
Available-for Sale, Fair Value of Investments with Unrealized Losses, 12 Months or Greater | 70,306 | 5,888
Available-for Sale, Gross Unrealized Losses, 12 Months or Greater | 654 | 41
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total | 129,405 | 88,683
Available-for Sale, Gross Unrealized Losses, Total | 933 | 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 |  | 2,798
Held-to-maturity securities, Gross Unrealized Losses, Less Than 12 Months |  | 40
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, 12 Months or Greater | 2,287 |
Held-to-maturity securities, Gross Unrealized Losses, 12 Months or Greater | 80 |
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Total | 2,287 | 2,798
Held-to-maturity securities, Gross Unrealized Losses, Total | 80 | 40
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months |  | 14,686
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months |  | 59
Available-for Sale, Fair Value of Investments with Unrealized Losses, 12 Months or Greater | 12,515 | 3,328
Available-for Sale, Gross Unrealized Losses, 12 Months or Greater | 320 | 740
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total | 12,515 | 18,014
Available-for Sale, Gross Unrealized Losses, Total | 320 | 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 | 2,865 | 48,153
Held-to-maturity securities, Gross Unrealized Losses, Less Than 12 Months | 4 | 122
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, 12 Months or Greater | 46,486 | 3,948
Held-to-maturity securities, Gross Unrealized Losses, 12 Months or Greater | 335 | 76
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Total | 49,351 | 52,101
Held-to-maturity securities, Gross Unrealized Losses, Total | 339 | 198
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months |  | 14,730
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months |  | 204
Available-for Sale, Fair Value of Investments with Unrealized Losses, 12 Months or Greater | 59,756 | 47,716
Available-for Sale, Gross Unrealized Losses, 12 Months or Greater | 1,429 | 922
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total | 59,756 | 62,446
Available-for Sale, Gross Unrealized Losses, Total | 1,429 | 1,126
Asset-Backed Securities [Member] |  |
Investment [Line Items] |  |
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, 12 Months or Greater | 525 | 967
Held-to-maturity securities, Gross Unrealized Losses, 12 Months or Greater | 8 | 15
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Total | 525 | 967
Held-to-maturity securities, Gross Unrealized Losses, Total | $ 8 | $ 15

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