Investments - Other Investments (Details) - USD ($) $ in Millions	1 Months Ended	12 Months Ended
Mar. 31, 2021	Dec. 31, 2023	Dec. 31, 2022
Equity Method Investment, Aggregate Cost [Roll Forward]
Opening undistributed value of investment, beginning balance		$ 6.2	$ 3.9
Investment in the period		0.4	2.0
Proceeds from Sale of Equity Method Investments		0.0	0.0
Closing value of investment, ending balance		7.6	6.2
Fair Value, Recurring [Member]
Equity Method Investment, Aggregate Cost [Roll Forward]
Investments, fair value		117.1	134.6
Equity Method Investment, Nonconsolidated Investee, Other
Equity Method Investment, Aggregate Cost [Roll Forward]
Unrealized Gain (Loss) on Investments		1.0	0.3
MVI
Equity Method Investment, Aggregate Cost [Roll Forward]
Opening undistributed value of investment, beginning balance		0.8	0.5
Investment in the period	$ 0.8	0.0	0.4
Proceeds from Sale of Equity Method Investments		0.0	0.0
Closing value of investment, ending balance		1.0	0.8
MVI \| Equity Method Investment, Nonconsolidated Investee, Other
Equity Method Investment, Aggregate Cost [Roll Forward]
Unrealized Gain (Loss) on Investments		0.2	(0.1)
Multi-Line Insurer
Equity Method Investment, Aggregate Cost [Roll Forward]
Opening undistributed value of investment, beginning balance		5.2	3.2
Investment in the period		0.4	1.6
Proceeds from Sale of Equity Method Investments		0.0	0.0
Closing value of investment, ending balance		6.4	5.2
Multi-Line Insurer \| Equity Method Investment, Nonconsolidated Investee, Other
Equity Method Investment, Aggregate Cost [Roll Forward]
Unrealized Gain (Loss) on Investments		0.8	0.4
Digital Re
Equity Method Investment, Aggregate Cost [Roll Forward]
Opening undistributed value of investment, beginning balance		0.2	0.2
Investment in the period		0.0	0.0
Proceeds from Sale of Equity Method Investments		0.0	0.0
Closing value of investment, ending balance		0.2	0.2
Digital Re \| Equity Method Investment, Nonconsolidated Investee, Other
Equity Method Investment, Aggregate Cost [Roll Forward]
Unrealized Gain (Loss) on Investments		$ 0.0	$ 0.0

Equity Method Investment, Aggregate Cost [Roll Forward]

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- Definition

This element represents the aggregate cost of investments accounted for under the equity method of accounting.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.12)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480566/210-10-S99-1

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- Definition

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
-Topic 820
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482106/820-10-50-2

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- Definition

The net cash outflow or inflow associated with the acquisition or sale of a business segment during the period.

+ References

No definition available.

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- Definition

The cash inflow associated with the sale of equity method investments, which are investments in joint ventures and entities in which the entity has an equity ownership interest normally of 20 to 50 percent and exercises significant influence.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 12
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482740/230-10-45-12

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- Definition

Amount of unrealized gain (loss) on investment.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Section 45
-Paragraph 28
-Subparagraph (b)
-SubTopic 10
-Topic 230
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482740/230-10-45-28

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