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Fair Value (Narrative) (Details) - USD ($) $ in Millions | 3 Months Ended | 12 Months Ended
--- | --- | ---
Jun. 30, 2020 | Mar. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2020 | Dec. 31, 2019
Fair Value Disclosure [Line Items] |  |  |  |  |
Transfers between Fair Value Hierarchies |  |  |  | $ 0.0 | $ 0.0
Allowance for Credit Loss |  |  | $ 0.0 | (0.5) | 0.0
Disposal Group, Not Discontinued Operation, Loss (Gain) on Write-down |  |  |  | 412.4 |
U.S. Treasury debt securities |  |  |  |  |
Fair Value Disclosure [Line Items] |  |  |  |  |
Allowance for Credit Loss |  |  | 0.0 | 0.0 | 0.0
Available-for-sale securities, maturities of less than a year |  |  |  | 4.9 |
Corporate/Other debt securities |  |  |  |  |
Fair Value Disclosure [Line Items] |  |  |  |  |
Allowance for Credit Loss |  |  | 0.0 | (0.5) | $ 0.0
Available-for-sale securities, maturities of less than a year |  |  |  | $ 4.3 |
Columbia Of Massachusetts |  |  |  |  |
Fair Value Disclosure [Line Items] |  |  |  |  |
Disposal Group, Not Discontinued Operation, Loss (Gain) on Write-down | $ 84.4 | $ 280.2 |  |  |
Goodwill, Impairment Loss |  |  | 204.8 |  |
Impairment of Intangible Assets, Finite-lived |  |  | $ 209.7 |  |

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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 next 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 -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 320 -Section 50 -Paragraph 3A -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=120252992&loc=SL120269850-112803 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 3 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=121645371&loc=d3e27198-111563 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=121645371&loc=d3e27161-111563 + Details
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