Fair Value Disclosures - Carrying Value and Fair Value of the Financial Instruments that are Not Recognized or are Not Carried at Fair Value (Details) - Recurring - USD ($) $ in Millions	Sep. 30, 2022	Dec. 31, 2021
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total financial assets	$ 7,405.9	$ 9,198.2
Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total financial assets	888.5	1,519.9
Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total financial assets	6,453.2	7,536.2
Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total financial assets	64.2	142.1
Carrying Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Commercial mortgage loans on real estate	304.4	256.5
Other investments	6.8	4.2
Other assets	13.8	24.9
Total financial assets	325.0	285.6
Policy reserves under investment products (Individual and group annuities, subject to discretionary withdrawal)	8.1	8.5
Funds withheld under reinsurance	374.5	364.2
Debt	2,129.3	2,202.5
Total financial liabilities	2,511.9	2,575.2
Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Commercial mortgage loans on real estate	284.5	266.0
Other investments	6.8	4.2
Other assets	13.8	24.9
Total financial assets	305.1	295.1
Policy reserves under investment products (Individual and group annuities, subject to discretionary withdrawal)	8.2	9.6
Funds withheld under reinsurance	374.5	364.2
Debt	1,938.7	2,456.3
Total financial liabilities	2,321.4	2,830.1
Fair Value \| Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Commercial mortgage loans on real estate	0.0	0.0
Other investments	1.7	2.0
Other assets	0.0	0.0
Total financial assets	1.7	2.0
Policy reserves under investment products (Individual and group annuities, subject to discretionary withdrawal)	0.0	0.0
Funds withheld under reinsurance	374.5	364.2
Debt	0.0	0.0
Total financial liabilities	374.5	364.2
Fair Value \| Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Commercial mortgage loans on real estate	0.0	0.0
Other investments	0.0	0.0
Other assets	0.0	0.0
Total financial assets	0.0	0.0
Policy reserves under investment products (Individual and group annuities, subject to discretionary withdrawal)	0.0	0.0
Funds withheld under reinsurance	0.0	0.0
Debt	1,938.7	2,456.3
Total financial liabilities	1,938.7	2,456.3
Fair Value \| Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Commercial mortgage loans on real estate	284.5	266.0
Other investments	5.1	2.2
Other assets	13.8	24.9
Total financial assets	303.4	293.1
Policy reserves under investment products (Individual and group annuities, subject to discretionary withdrawal)	8.2	9.6
Funds withheld under reinsurance	0.0	0.0
Debt	0.0	0.0
Total financial liabilities	$ 8.2	$ 9.6

Funds Held under Reinsurance Agreements, Liability, Fair Value Disclosure

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Policy Reserves Under Investment Products, Fair Value Disclosure

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No definition available.

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- Definition

Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
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-Topic 820
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-Paragraph 2
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Fair value portion of debt instrument payable, including, but not limited to, notes payable and loans payable.

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Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 20
-Section 55
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-Publisher FASB
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Fair value portion of investment securities, including, but not limited to, marketable securities, derivative financial instruments, and investments accounted for under the equity method.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI https://asc.fasb.org/extlink&oid=126976982&loc=d3e19207-110258

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Fair value of financial and nonfinancial obligations.

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Fair value portion of mortgage loans held-for-sale.

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Fair value portion of other assets.

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