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Marketable Securities Held in Trusts - Maturity Dates and Realized Gain and Loss (Details) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended |
--- | --- | --- | ---
Sep. 30, 2024 | Sep. 30, 2023 | Sep. 30, 2024 | Sep. 30, 2023 | Dec. 31, 2023
Available-for-sale Securities, Debt Maturities, Fair Value, Fiscal Year Maturity |  |  |  |  |
Debt Securities, Available-for-sale | $ 713.2 |  | $ 713.2 |  | $ 683.6
Debt securities |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |
Debt Securities, Available-for-sale, Realized Loss | 1.2 | $ 5.7 | 10.4 | $ 20.9 |
Debt Securities, Available-for-sale, Realized Gain | 6.4 | $ 0.1 | 17.2 | $ 9.2 |
Available-for-sale Securities, Debt Maturities, Fair Value, Fiscal Year Maturity |  |  |  |  |
Available-for-sale debt maturities due within one year, fair value | 24.1 |  | 24.1 |  |
Available-for-sale debt maturities, after 1 but within 5 years, fair value | 275.3 |  | 275.3 |  |
Available-for-sale debt maturities, after 5 but within 10 years, fair value | 350.4 |  | 350.4 |  |
Available-for-sale debt maturities, after 10 years, fair value | 61.5 |  | 61.5 |  |
Debt Securities, Available-for-sale | $ 711.3 |  | $ 711.3 |  |

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- Definition Fair value of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), with single maturity date and allocated without single maturity date, maturing in sixth through tenth fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach). + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 320 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 3 -Subparagraph (c) -Publisher FASB -URI https://asc.fasb.org/1943274/2147481800/320-10-50-3 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 942 -SubTopic 320 -Name Accounting Standards Codification -Section 50 -Paragraph 3A -Subparagraph (c) -Publisher FASB -URI https://asc.fasb.org/1943274/2147477268/942-320-50-3A + Details
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- Definition Fair value of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), with single maturity date and allocated without single maturity date, maturing in second through fifth fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach). + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 320 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 3 -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org/1943274/2147481800/320-10-50-3 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 942 -SubTopic 320 -Name Accounting Standards Codification -Section 50 -Paragraph 3A -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org/1943274/2147477268/942-320-50-3A + Details
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