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Investments - Schedule of Amortized Cost Held-to-Maturity Securities Aggregated By Credit Quality Indicator (Detail) $ in Thousands | Sep. 30, 2020 USD ($)
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Debt Securities Held To Maturity Credit Quality Indicator [Line Items] |
Amortized cost | $ 572,796
AAA/AA/A Ratings [Member] |
Debt Securities Held To Maturity Credit Quality Indicator [Line Items] |
Amortized cost | 529,678
Baa/BBB Ratings [Member] |
Debt Securities Held To Maturity Credit Quality Indicator [Line Items] |
Amortized cost | 43,082
B Ratings [Member] |
Debt Securities Held To Maturity Credit Quality Indicator [Line Items] |
Amortized cost | 36
States and Political Subdivisions [Member] |
Debt Securities Held To Maturity Credit Quality Indicator [Line Items] |
Amortized cost | 471,913
States and Political Subdivisions [Member] | AAA/AA/A Ratings [Member] |
Debt Securities Held To Maturity Credit Quality Indicator [Line Items] |
Amortized cost | 469,485
States and Political Subdivisions [Member] | Baa/BBB Ratings [Member] |
Debt Securities Held To Maturity Credit Quality Indicator [Line Items] |
Amortized cost | 2,428
Corporate Bonds [Member] |
Debt Securities Held To Maturity Credit Quality Indicator [Line Items] |
Amortized cost | 70,721
Corporate Bonds [Member] | AAA/AA/A Ratings [Member] |
Debt Securities Held To Maturity Credit Quality Indicator [Line Items] |
Amortized cost | 30,085
Corporate Bonds [Member] | Baa/BBB Ratings [Member] |
Debt Securities Held To Maturity Credit Quality Indicator [Line Items] |
Amortized cost | 40,636
U.S. Agency-Based Mortgage-Backed Securities [Member] |
Debt Securities Held To Maturity Credit Quality Indicator [Line Items] |
Amortized cost | 8,070
U.S. Agency-Based Mortgage-Backed Securities [Member] | AAA/AA/A Ratings [Member] |
Debt Securities Held To Maturity Credit Quality Indicator [Line Items] |
Amortized cost | 8,070
U.S. Treasury Securities and Obligations of U.S. Government Agencies [Member] |
Debt Securities Held To Maturity Credit Quality Indicator [Line Items] |
Amortized cost | 21,914
U.S. Treasury Securities and Obligations of U.S. Government Agencies [Member] | AAA/AA/A Ratings [Member] |
Debt Securities Held To Maturity Credit Quality Indicator [Line Items] |
Amortized cost | 21,914
Asset-Backed Securities [Member] |
Debt Securities Held To Maturity Credit Quality Indicator [Line Items] |
Amortized cost | 178
Asset-Backed Securities [Member] | AAA/AA/A Ratings [Member] |
Debt Securities Held To Maturity Credit Quality Indicator [Line Items] |
Amortized cost | 124
Asset-Backed Securities [Member] | Baa/BBB Ratings [Member] |
Debt Securities Held To Maturity Credit Quality Indicator [Line Items] |
Amortized cost | 18
Asset-Backed Securities [Member] | B Ratings [Member] |
Debt Securities Held To Maturity Credit Quality Indicator [Line Items] |
Amortized cost | $ 36

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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
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- Definition Amount of investment in debt security measured at amortized cost (held-to-maturity) originated in current fiscal year. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 326 -SubTopic 20 -Section 50 -Paragraph 6 -URI http://asc.fasb.org/extlink&oid=121599337&loc=SL82919232-210447 + Details
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