Investments - Composition of the Fair Values of Investments by Credit Rating (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
Mar. 31, 2025	Dec. 31, 2024
Debt Securities, Held-to-Maturity, Credit Quality Indicator [Line Items]
Fair value		$ 3,411.6
Fixed maturity securities
Debt Securities, Held-to-Maturity, Credit Quality Indicator [Line Items]
Fair value	$ 3,080.5	$ 3,411.6
Fixed maturity securities \| Credit Rating \| Credit Concentration Risk
Debt Securities, Held-to-Maturity, Credit Quality Indicator [Line Items]
Concentration risk, percentage	100.00%	100.00%
Short-term investments
Debt Securities, Held-to-Maturity, Credit Quality Indicator [Line Items]
Fair value	$ 144.4	$ 222.1
Short-term investments \| Credit Rating \| Credit Concentration Risk
Debt Securities, Held-to-Maturity, Credit Quality Indicator [Line Items]
Concentration risk, percentage	100.00%	100.00%
AAA \| Fixed maturity securities
Debt Securities, Held-to-Maturity, Credit Quality Indicator [Line Items]
Fair value	$ 299.2	$ 399.4
AAA \| Fixed maturity securities \| Credit Rating \| Credit Concentration Risk
Debt Securities, Held-to-Maturity, Credit Quality Indicator [Line Items]
Concentration risk, percentage	10.00%	12.00%
AAA \| Short-term investments
Debt Securities, Held-to-Maturity, Credit Quality Indicator [Line Items]
Fair value	$ 0.3	$ 0.3
AAA \| Short-term investments \| Credit Rating \| Credit Concentration Risk
Debt Securities, Held-to-Maturity, Credit Quality Indicator [Line Items]
Concentration risk, percentage	0.00%	0.00%
AA \| Fixed maturity securities
Debt Securities, Held-to-Maturity, Credit Quality Indicator [Line Items]
Fair value	$ 996.9	$ 1,245.6
AA \| Fixed maturity securities \| Credit Rating \| Credit Concentration Risk
Debt Securities, Held-to-Maturity, Credit Quality Indicator [Line Items]
Concentration risk, percentage	32.00%	37.00%
AA \| Short-term investments
Debt Securities, Held-to-Maturity, Credit Quality Indicator [Line Items]
Fair value	$ 143.6	$ 220.7
AA \| Short-term investments \| Credit Rating \| Credit Concentration Risk
Debt Securities, Held-to-Maturity, Credit Quality Indicator [Line Items]
Concentration risk, percentage	100.00%	100.00%
A \| Fixed maturity securities
Debt Securities, Held-to-Maturity, Credit Quality Indicator [Line Items]
Fair value	$ 1,242.9	$ 1,270.9
A \| Fixed maturity securities \| Credit Rating \| Credit Concentration Risk
Debt Securities, Held-to-Maturity, Credit Quality Indicator [Line Items]
Concentration risk, percentage	40.00%	37.00%
BBB \| Fixed maturity securities
Debt Securities, Held-to-Maturity, Credit Quality Indicator [Line Items]
Fair value	$ 463.0	$ 453.1
BBB \| Fixed maturity securities \| Credit Rating \| Credit Concentration Risk
Debt Securities, Held-to-Maturity, Credit Quality Indicator [Line Items]
Concentration risk, percentage	15.00%	13.00%
BBB \| Short-term investments
Debt Securities, Held-to-Maturity, Credit Quality Indicator [Line Items]
Fair value	$ 0.0	$ 0.2
BBB \| Short-term investments \| Credit Rating \| Credit Concentration Risk
Debt Securities, Held-to-Maturity, Credit Quality Indicator [Line Items]
Concentration risk, percentage	0.00%	0.00%
Below BBB \| Fixed maturity securities
Debt Securities, Held-to-Maturity, Credit Quality Indicator [Line Items]
Fair value	$ 78.5	$ 42.6
Below BBB \| Fixed maturity securities \| Credit Rating \| Credit Concentration Risk
Debt Securities, Held-to-Maturity, Credit Quality Indicator [Line Items]
Concentration risk, percentage	3.00%	1.00%
Below BBB \| Short-term investments
Debt Securities, Held-to-Maturity, Credit Quality Indicator [Line Items]
Fair value	$ 0.5	$ 0.9
Below BBB \| Short-term investments \| Credit Rating \| Credit Concentration Risk
Debt Securities, Held-to-Maturity, Credit Quality Indicator [Line Items]
Concentration risk, percentage	0.00%	0.00%

For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 280
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
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-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482810/280-10-50-42

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Topic 825
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-Section 50
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-Publisher FASB
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Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
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Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
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-Topic 275
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Reference 5: http://fasb.org/us-gaap/role/ref/legacyRef
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Amount excluding accrued interest, of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 326
-SubTopic 30
-Name Accounting Standards Codification
-Section 45
-Paragraph 1
-Publisher FASB
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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.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
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-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479319/326-20-50-5

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