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Investment Securities - Debt securities issued by corporation (Details) - Debt securities issued by corporations - USD ($) $ in Millions | Jun. 30, 2023 | Dec. 31, 2022
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Debt and Equity Securities, FV-NI [Line Items] |  |
Fair value investments | $ 992.5 | $ 1,018.8
AAA |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Fair value investments | 12.7 | 11.3
AA |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Fair value investments | 87.7 | 96.0
A |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Fair value investments | 552.4 | 567.9
BBB |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Fair value investments | 333.4 | 337.7
Other |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Fair value investments | $ 6.3 | $ 5.9

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- Definition Fair value portion of investment securities, including, but not limited to, marketable securities, derivative financial instruments, and investments accounted for under the equity method. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org//1943274/2147482106/820-10-50-2 + Details
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