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Investments - Gross Unrecognized Gains and Losses and Cost or Amortized Cost, Allowance for Credit Losses and Fair Value of Investments Classified as Held-to-Maturity (Detail) - USD ($) $ in Thousands | Mar. 31, 2020 | Dec. 31, 2019
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Schedule Of Held To Maturity Securities [Line Items] |  |
Held-to-maturity, Amortized Cost, Totals | $ 608,590 | $ 599,421
Held-to-Maturity, Gross Unrecognized Gains | 24,308 | 22,128
Held-to-Maturity, Gross Unrecognized Losses | (935) | (206)
Held-to-Maturity, Allowance for Credit Losses | (275) | 0
Held-to-Maturity, Fair Value | 631,688 | 621,343
States and Political Subdivisions [Member] |  |
Schedule Of Held To Maturity Securities [Line Items] |  |
Held-to-maturity, Amortized Cost, Totals | 483,340 | 466,270
Held-to-Maturity, Gross Unrecognized Gains | 21,875 | 19,570
Held-to-Maturity, Gross Unrecognized Losses | (145) | (193)
Held-to-Maturity, Allowance for Credit Losses | (50) |
Held-to-Maturity, Fair Value | 505,020 | 485,647
Corporate Bonds [Member] |  |
Schedule Of Held To Maturity Securities [Line Items] |  |
Held-to-maturity, Amortized Cost, Totals | 100,969 | 109,241
Held-to-Maturity, Gross Unrecognized Gains | 1,200 | 1,684
Held-to-Maturity, Gross Unrecognized Losses | (790) |
Held-to-Maturity, Allowance for Credit Losses | (215) |
Held-to-Maturity, Fair Value | 101,164 | 110,925
U.S. Agency-Based Mortgage-Backed Securities [Member] |  |
Schedule Of Held To Maturity Securities [Line Items] |  |
Held-to-maturity, Amortized Cost, Totals | 10,352 | 10,967
Held-to-Maturity, Gross Unrecognized Gains | 720 | 544
Held-to-Maturity, Fair Value | 11,072 | 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 | 13,721 | 12,723
Held-to-Maturity, Gross Unrecognized Gains | 513 | 330
Held-to-Maturity, Gross Unrecognized Losses |  | (12)
Held-to-Maturity, Fair Value | 14,234 | 13,041
Asset-Backed Securities [Member] |  |
Schedule Of Held To Maturity Securities [Line Items] |  |
Held-to-maturity, Amortized Cost, Totals | 208 | 220
Held-to-Maturity, Gross Unrecognized Losses |  | (1)
Held-to-Maturity, Allowance for Credit Losses | (10) |
Held-to-Maturity, Fair Value | $ 198 | $ 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=119407570&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=118939692&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=118955202&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=119407570&loc=SL82919249-210447 + Details
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- Definition Amount of accumulated unrecognized gain on investment in debt security measured at amortized cost (held-to-maturity). + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 5 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=118939692&loc=d3e27232-111563 + Details
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