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Investments - Gross Unrecognized Gains and Losses and Cost or Amortized Cost, Allowance for Credit Losses, Carrying Amount and Fair Value of Investments Classified as Held-to-Maturity (Detail) - USD ($) $ in Thousands | Mar. 31, 2021 | Dec. 31, 2020
--- | --- | ---
Schedule Of Held To Maturity Securities [Line Items] |  |
Held-to-maturity, Amortized Cost, Totals | $ 575,274 | $ 585,404
Held-to-Maturity, Allowance for Credit Losses | (166) | (274)
Held-to-Maturity, Carrying Amount | 575,108 | 585,130
Held-to-Maturity, Gross Unrecognized Gains | 29,388 | 36,524
Held-to-Maturity, Gross Unrecognized Losses | (776) |
Held-to-Maturity, Fair Value | 603,720 | 621,654
States and Political Subdivisions [Member] |  |
Schedule Of Held To Maturity Securities [Line Items] |  |
Held-to-maturity, Amortized Cost, Totals | 486,376 | 494,374
Held-to-Maturity, Allowance for Credit Losses | (44) | (42)
Held-to-Maturity, Carrying Amount | 486,332 | 494,332
Held-to-Maturity, Gross Unrecognized Gains | 26,179 | 32,489
Held-to-Maturity, Gross Unrecognized Losses | (774) |
Held-to-Maturity, Fair Value | 511,737 | 526,821
Corporate Bonds [Member] |  |
Schedule Of Held To Maturity Securities [Line Items] |  |
Held-to-maturity, Amortized Cost, Totals | 68,518 | 69,981
Held-to-Maturity, Allowance for Credit Losses | (116) | (225)
Held-to-Maturity, Carrying Amount | 68,402 | 69,756
Held-to-Maturity, Gross Unrecognized Gains | 2,440 | 3,144
Held-to-Maturity, Fair Value | 70,842 | 72,900
U.S. Agency-Based Mortgage-Backed Securities [Member] |  |
Schedule Of Held To Maturity Securities [Line Items] |  |
Held-to-maturity, Amortized Cost, Totals | 6,357 | 7,261
Held-to-Maturity, Carrying Amount | 6,357 | 7,261
Held-to-Maturity, Gross Unrecognized Gains | 568 | 645
Held-to-Maturity, Fair Value | 6,925 | 7,906
U.S. Treasury Securities and Obligations of U.S. Government Agencies [Member] |  |
Schedule Of Held To Maturity Securities [Line Items] |  |
Held-to-maturity, Amortized Cost, Totals | 13,877 | 13,626
Held-to-Maturity, Carrying Amount | 13,877 | 13,626
Held-to-Maturity, Gross Unrecognized Gains | 195 | 239
Held-to-Maturity, Gross Unrecognized Losses | (2) |
Held-to-Maturity, Fair Value | 14,070 | 13,865
Asset-Backed Securities [Member] |  |
Schedule Of Held To Maturity Securities [Line Items] |  |
Held-to-maturity, Amortized Cost, Totals | 146 | 162
Held-to-Maturity, Allowance for Credit Losses | (6) | (7)
Held-to-Maturity, Carrying Amount | 140 | 155
Held-to-Maturity, Gross Unrecognized Gains | 6 | 7
Held-to-Maturity, Fair Value | $ 146 | $ 162

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- Definition Amount of allowance for credit loss for debt security measured at amortized cost (held-to-maturity). + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 326 -SubTopic 20 -Section 50 -Paragraph 13 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=121599337&loc=SL82919249-210447 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 5 -Subparagraph (aaa) -URI http://asc.fasb.org/extlink&oid=121645371&loc=d3e27232-111563 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 326 -SubTopic 20 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=121599878&loc=SL82895884-210446 Reference 4: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 326 -SubTopic 20 -Section 50 -Paragraph 13 -Subparagraph (f) -URI http://asc.fasb.org/extlink&oid=121599337&loc=SL82919249-210447 + Details
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