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Investment Securities - Investment Holding Common Equities/ Other Investments (Details) - USD ($) $ in Millions | 9 Months Ended | 12 Months Ended
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Sep. 30, 2020 | Dec. 31, 2019
Common equity securities |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Cost or Amortized Cost | $ 124.2 | $ 553.3
Gross Unrealized Gains | 16.5 | 130.6
Gross Unrealized Losses | 0.0 | 0.0
Net Foreign Currency Losses | 0.0 | 0.0
Common equity securities, at fair value | 140.7 | 683.9
Other long-term investments |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Cost or Amortized Cost | 696.4 | 667.4
Gross Unrealized Gains | 552.4 | 255.2
Gross Unrealized Losses | (76.1) | (64.1)
Net Foreign Currency Losses | (1.5) | (2.2)
Common equity securities, at fair value | $ 1,171.2 | $ 856.3

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- Definition Amount of investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI). + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 45 -Paragraph 1A -URI http://asc.fasb.org/extlink&oid=99377092&loc=SL75136599-209740 + Details
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