Investments - Composition of the Fair Values of Invetments by Credit Rating (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2023	Dec. 31, 2022
Credit Rating \| Credit Concentration Risk
Debt Securities, Held-to-Maturity, Credit Quality Indicator [Line Items]
Concentration Risk, Percentage	100.00%	100.00%
Wellington Funds \| Credit Rating \| Credit Concentration Risk
Debt Securities, Held-to-Maturity, Credit Quality Indicator [Line Items]
Concentration Risk, Percentage	99.00%	37.00%
York Fund \| Credit Rating \| Credit Concentration Risk
Debt Securities, Held-to-Maturity, Credit Quality Indicator [Line Items]
Concentration Risk, Percentage	1.00%	1.00%
Equity and commodity structured notes \| Credit Rating \| Credit Concentration Risk
Debt Securities, Held-to-Maturity, Credit Quality Indicator [Line Items]
Concentration Risk, Percentage	0.00%	62.00%
Fixed maturity securities
Debt Securities, Held-to-Maturity, Credit Quality Indicator [Line Items]
Fair Value	$ 3,244.9	$ 2,050.9
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	$ 49.0	$ 257.0
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	$ 399.5	$ 269.1
AAA \| Fixed maturity securities \| Credit Rating \| Credit Concentration Risk
Debt Securities, Held-to-Maturity, Credit Quality Indicator [Line Items]
Concentration Risk, Percentage	12.00%	13.00%
AAA \| Short-Term Investments
Debt Securities, Held-to-Maturity, Credit Quality Indicator [Line Items]
Fair Value	$ 0.0	$ 23.1
AAA \| Short-Term Investments \| Credit Rating \| Credit Concentration Risk
Debt Securities, Held-to-Maturity, Credit Quality Indicator [Line Items]
Concentration Risk, Percentage	0.00%	9.00%
AA \| Fixed maturity securities
Debt Securities, Held-to-Maturity, Credit Quality Indicator [Line Items]
Fair Value	$ 1,119.0	$ 775.6
AA \| Fixed maturity securities \| Credit Rating \| Credit Concentration Risk
Debt Securities, Held-to-Maturity, Credit Quality Indicator [Line Items]
Concentration Risk, Percentage	35.00%	38.00%
AA \| Short-Term Investments
Debt Securities, Held-to-Maturity, Credit Quality Indicator [Line Items]
Fair Value	$ 49.0	$ 232.9
AA \| Short-Term Investments \| Credit Rating \| Credit Concentration Risk
Debt Securities, Held-to-Maturity, Credit Quality Indicator [Line Items]
Concentration Risk, Percentage	100.00%	91.00%
A \| Fixed maturity securities
Debt Securities, Held-to-Maturity, Credit Quality Indicator [Line Items]
Fair Value	$ 1,145.8	$ 622.3
A \| Fixed maturity securities \| Credit Rating \| Credit Concentration Risk
Debt Securities, Held-to-Maturity, Credit Quality Indicator [Line Items]
Concentration Risk, Percentage	35.00%	30.00%
A \| Short-Term Investments
Debt Securities, Held-to-Maturity, Credit Quality Indicator [Line Items]
Fair Value	$ 0.0	$ 1.0
A \| 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%
BBB \| Fixed maturity securities
Debt Securities, Held-to-Maturity, Credit Quality Indicator [Line Items]
Fair Value	$ 573.6	$ 383.9
BBB \| Fixed maturity securities \| Credit Rating \| Credit Concentration Risk
Debt Securities, Held-to-Maturity, Credit Quality Indicator [Line Items]
Concentration Risk, Percentage	18.00%	19.00%
BB \| Fixed maturity securities
Debt Securities, Held-to-Maturity, Credit Quality Indicator [Line Items]
Fair Value	$ 7.0	$ 0.0
BB \| Fixed maturity securities \| 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
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Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
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Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
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Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
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Reference 5: http://fasb.org/us-gaap/role/ref/legacyRef
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+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 326
-SubTopic 30
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Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
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