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Investments - Investment Securities Continuous Unrealized Loss Position (Detail) - USD ($) $ in Thousands | Sep. 30, 2020 | Dec. 31, 2019
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Marketable Securities [Line Items] |  |
Held-to-Maturity, Fair Value of Investments with Unrealized Losses, Less Than 12 Months |  | $ 21,074
Held-to-Maturity, Gross Unrealized Losses, Less Than 12 Months |  | 193
Held-to-Maturity, Fair Value of Investments with Unrealized Losses, 12 Months or Greater |  | 3,311
Held-to-Maturity, Gross Unrealized Losses, 12 Months or Greater |  | 13
Held-to-Maturity, Fair Value of Investments with Unrealized Losses, Total |  | 24,385
Held-to-Maturity, Gross Unrealized Losses, Total | $ 8 | 206
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months | 6,840 | 23,573
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months | 58 | 122
Available-for Sale, Fair Value of Investments with Unrealized Losses, 12 Months or Greater |  | 18,220
Available-for Sale, Gross Unrealized Losses, 12 Months or Greater |  | 43
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total | 6,840 | 41,793
Available-for Sale, Gross Unrealized Losses, Total | 58 | 165
Fair Value of Investments with Unrealized Losses, Less Than 12 months |  | 44,647
Gross Unrealized Loss, Less Than 12 Months |  | 315
Fair Value of Investments with Unrealized Losses, 12 Months or Greater |  | 21,531
Gross Unrealized Losses, 12 Months or Greater |  | 56
Fair Value of Investments with Unrealized Losses, Total |  | 66,178
Gross Unrealized Losses, Total |  | 371
States and Political Subdivisions [Member] |  |
Marketable Securities [Line Items] |  |
Held-to-Maturity, Fair Value of Investments with Unrealized Losses, Less Than 12 Months |  | 21,074
Held-to-Maturity, Gross Unrealized Losses, Less Than 12 Months |  | 193
Held-to-Maturity, Fair Value of Investments with Unrealized Losses, Total |  | 21,074
Held-to-Maturity, Gross Unrealized Losses, Total | 8 | 193
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months | 4,329 | 4,140
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months | 33 | 26
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total | 4,329 | 4,140
Available-for Sale, Gross Unrealized Losses, Total | 33 | 26
Corporate Bonds [Member] |  |
Marketable Securities [Line Items] |  |
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months | 2,511 | 6,426
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months | 25 | 1
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total | 2,511 | 6,426
Available-for Sale, Gross Unrealized Losses, Total | $ 25 | 1
U.S. Treasury Securities and Obligations of U.S. Government Agencies [Member] |  |
Marketable Securities [Line Items] |  |
Held-to-Maturity, Fair Value of Investments with Unrealized Losses, 12 Months or Greater |  | 3,243
Held-to-Maturity, Gross Unrealized Losses, 12 Months or Greater |  | 12
Held-to-Maturity, Fair Value of Investments with Unrealized Losses, Total |  | 3,243
Held-to-Maturity, Gross Unrealized Losses, Total |  | 12
Available-for Sale, Fair Value of Investments with Unrealized Losses, 12 Months or Greater |  | 17,068
Available-for Sale, Gross Unrealized Losses, 12 Months or Greater |  | 42
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total |  | 17,068
Available-for Sale, Gross Unrealized Losses, Total |  | 42
Asset-Backed Securities [Member] |  |
Marketable Securities [Line Items] |  |
Held-to-Maturity, Fair Value of Investments with Unrealized Losses, 12 Months or Greater |  | 68
Held-to-Maturity, Gross Unrealized Losses, 12 Months or Greater |  | 1
Held-to-Maturity, Fair Value of Investments with Unrealized Losses, Total |  | 68
Held-to-Maturity, Gross Unrealized Losses, Total |  | 1
U.S. Agency-Based Mortgage-Backed Securities [Member] |  |
Marketable Securities [Line Items] |  |
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months |  | 13,007
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months |  | 95
Available-for Sale, Fair Value of Investments with Unrealized Losses, 12 Months or Greater |  | 1,152
Available-for Sale, Gross Unrealized Losses, 12 Months or Greater |  | 1
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total |  | 14,159
Available-for Sale, Gross Unrealized Losses, Total |  | $ 96

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