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Marketable Securities Held in Trusts - Continuous Loss Position (Details) - USD ($) $ in Millions | Sep. 30, 2021 | Dec. 31, 2020
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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 | $ 53.6 | $ 4.6
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | (1.3) | (0.1)
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months | 333.8 | 137.8
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 4.3 | 0.3
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 | 0.0 | 0.0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | 0.0 | 0.0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months | 39.3 | 1.5
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 0.4 | 0.0
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 | 5.3 | 4.6
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | (0.1) | (0.1)
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months | 41.2 | 16.0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 0.5 | 0.2
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 | 48.3 | 0.0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | (1.2) | 0.0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months | 253.3 | 120.3
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | $ 3.4 | $ 0.1

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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 http://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 http://asc.fasb.org/extlink&oid=123581744&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 http://asc.fasb.org/extlink&oid=121590138&loc=SL82922954-210456 + Details
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