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Marketable Securities Held in Trusts - Continuous Loss Position (Details) - USD ($) $ in Millions | Mar. 31, 2022 | Dec. 31, 2021
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Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss |  |
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value | $ 182.5 | $ 151.5
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | (14.8) | (3.8)
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months | 387.5 | 259.2
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 19.9 | 3.7
Corporate debt securities |  |
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss |  |
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value | 6.5 | 3.6
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | (0.7) | (0.1)
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months | 143.8 | 67.1
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 6.6 | 0.8
Municipal bonds |  |
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss |  |
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value | 10.3 | 4.5
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | (0.8) | (0.1)
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months | 127.2 | 39.9
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 5.0 | 0.4
U.S. government bonds |  |
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss |  |
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value | 165.7 | 143.4
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | (13.3) | (3.6)
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months | 116.5 | 152.2
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | $ 8.3 | $ 2.5

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- Definition Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for more than 12 months, without allowance for credit loss. Includes beneficial interest in securitized financial asset. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 326 -SubTopic 30 -Section 50 -Paragraph 5 -URI https://asc.fasb.org/extlink&oid=124269663&loc=SL82922890-210455 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 7 -URI https://asc.fasb.org/extlink&oid=126970911&loc=d3e27337-111563 Reference 3: http://www.xbrl.org/2003/role/exampleRef -Publisher FASB -Name Accounting Standards Codification -Topic 326 -SubTopic 30 -Section 55 -Paragraph 8 -URI https://asc.fasb.org/extlink&oid=121590138&loc=SL82922954-210456 + Details
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