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Investment Securities - Investment Holding Common Equities/ Other Investments/ MediaAlpha (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2021	Dec. 31, 2020
Common equity securities
Debt and Equity Securities, FV-NI [Line Items]
Cost or Amortized Cost	$ 117.6	$ 0.0
Gross Unrealized Gains	1.1	0.0
Gross Unrealized Losses	(0.2)	0.0
Net Foreign Currency Gains (Losses)	0.1	0.0
Common equity securities, at fair value	118.6	0.0
Marketing Technology (MediaAlpha)
Debt and Equity Securities, FV-NI [Line Items]
Cost or Amortized Cost	0.0	0.0
Gross Unrealized Gains	600.2	802.2
Gross Unrealized Losses	0.0	0.0
Net Foreign Currency Gains (Losses)	0.0	0.0
Common equity securities, at fair value	600.2	802.2
Other long-term investments
Debt and Equity Securities, FV-NI [Line Items]
Cost or Amortized Cost	965.5	767.4
Gross Unrealized Gains	125.8	95.8
Gross Unrealized Losses	(56.5)	(78.1)
Net Foreign Currency Gains (Losses)	(2.6)	1.7
Common equity securities, at fair value	$ 1,032.2	$ 786.8

X

- 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

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+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 321
-SubTopic 10
-Section 50
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=120269885&loc=SL75117546-209714

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+ References

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-Topic 321
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