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Investment Securities - Schedule of Interest Income on Fixed Maturity Investments (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Schedule of Investments [Line Items]
Total investment income	$ (20.1)	$ (71.5)	$ (58.8)	$ (114.6)
Third-party investment expenses	(0.6)	(0.3)	(1.6)	(0.9)
Net investment income	19.5	71.2	57.2	113.7
Fixed maturity investments
Schedule of Investments [Line Items]
Total investment income	(8.2)	(7.0)	(21.9)	(22.4)
Short-term investments
Schedule of Investments [Line Items]
Total investment income	0.0	(0.1)	(0.5)	(1.0)
Common equity securities
Schedule of Investments [Line Items]
Total investment income	(0.1)	(0.5)	(0.1)	(6.5)
Marketing Technology (MediaAlpha)
Schedule of Investments [Line Items]
Total investment income	0.0	(55.0)	0.0	(60.0)
Other long-term investments
Schedule of Investments [Line Items]
Total investment income	(12.0)	(8.9)	(37.1)	(24.7)
Amount attributable to TPC Providers
Schedule of Investments [Line Items]
Total investment income	$ (0.2)	$ 0.0	$ (0.8)	$ 0.0

X

- Definition

Represents the total of interest and dividend income, including any amortization and accretion (as applicable) of discounts and premiums, earned from (1) loans and leases whether held-for-sale or held-in-portfolio; (2) investment securities; (3) federal funds sold; (4) securities purchased under agreements to resell; (5) investments in banker's acceptances, commercial paper, or certificates of deposit; (6) dividend income; or (7) other investments not otherwise specified herein.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 220
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-04.1-5)
-URI http://asc.fasb.org/extlink&oid=120399700&loc=SL114874048-224260

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- Definition

Amount of expenses related to the generation of investment income.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.9)
-URI http://asc.fasb.org/extlink&oid=120395209&loc=SL114868664-224227

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 220
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-04.2)
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Amount, after investment expense, of income earned from investments in securities and real estate. Includes, but is not limited to, real estate investment, policy loans, dividends, and interest. Excludes realized gain (loss) on investments.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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-Name Accounting Standards Codification
-Topic 944
-SubTopic 220
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-04(2))
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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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