Fair value portion of investment securities, including, but not limited to, marketable securities, derivative financial instruments, and investments accounted for under the equity method.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
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Amount of investment in debt security measured at fair value with change in fair value recognized in net income (trading).

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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
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Investment Securities (Debt securities issued by corporation) (Details) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018	Sep. 30, 2018	Dec. 31, 2017
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	$ 2,569.4	$ 2,328.7	$ 2,365.1	$ 3,208.3
Debt securities issued by corporations
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	463.2	510.5
Fair value investments	463.2	510.5
AAA \| Debt securities issued by corporations
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	9.9	8.9
AA \| Debt securities issued by corporations
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	79.9	88.7
A \| Debt securities issued by corporations
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	286.7	270.5
BBB \| Debt securities issued by corporations
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	$ 86.7	$ 142.4