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Investments - Gross Unrealized Gains and Losses and Amortized Cost and Fair Value of Investments Classified as Held-to-Maturity (Detail) - USD ($) $ in Thousands | Dec. 31, 2020 | Dec. 31, 2019
--- | --- | ---
Schedule Of Held To Maturity Securities [Line Items] |  |
Held-to-maturity, Amortized Cost, Totals | $ 585,404 | $ 599,421
Held-to-Maturity, Allowance for Credit Losses | (274) | 0
Held-to-Maturity, Carrying Amount | 585,130 | 599,421
Held-to-Maturity, Gross Unrecognized Gains | 36,524 | 22,128
Held-to-Maturity, Gross Unrecognized Losses |  | (206)
Held-to-Maturity, Fair Value | 621,654 | 621,343
States and Political Subdivisions [Member] |  |
Schedule Of Held To Maturity Securities [Line Items] |  |
Held-to-maturity, Amortized Cost, Totals | 494,374 | 466,270
Held-to-Maturity, Allowance for Credit Losses | (42) |
Held-to-Maturity, Carrying Amount | 494,332 |
Held-to-Maturity, Gross Unrecognized Gains | 32,489 | 19,570
Held-to-Maturity, Gross Unrecognized Losses |  | (193)
Held-to-Maturity, Fair Value | 526,821 | 485,647
Corporate Bonds [Member] |  |
Schedule Of Held To Maturity Securities [Line Items] |  |
Held-to-maturity, Amortized Cost, Totals | 69,981 | 109,241
Held-to-Maturity, Allowance for Credit Losses | (225) |
Held-to-Maturity, Carrying Amount | 69,756 |
Held-to-Maturity, Gross Unrecognized Gains | 3,144 | 1,684
Held-to-Maturity, Fair Value | 72,900 | 110,925
U.S. Agency-Based Mortgage-Backed Securities [Member] |  |
Schedule Of Held To Maturity Securities [Line Items] |  |
Held-to-maturity, Amortized Cost, Totals | 7,261 | 10,967
Held-to-Maturity, Carrying Amount | 7,261 |
Held-to-Maturity, Gross Unrecognized Gains | 645 | 544
Held-to-Maturity, Fair Value | 7,906 | 11,511
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 | 13,626 | 12,723
Held-to-Maturity, Carrying Amount | 13,626 |
Held-to-Maturity, Gross Unrecognized Gains | 239 | 330
Held-to-Maturity, Gross Unrecognized Losses |  | (12)
Held-to-Maturity, Fair Value | 13,865 | 13,041
Asset-Backed Securities [Member] |  |
Schedule Of Held To Maturity Securities [Line Items] |  |
Held-to-maturity, Amortized Cost, Totals | 162 | 220
Held-to-Maturity, Allowance for Credit Losses | (7) |
Held-to-Maturity, Carrying Amount | 155 |
Held-to-Maturity, Gross Unrecognized Gains | 7 |
Held-to-Maturity, Gross Unrecognized Losses |  | (1)
Held-to-Maturity, Fair Value | $ 162 | $ 219

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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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