Investments - Schedule of Financial Assets Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Millions	Sep. 30, 2024	Dec. 31, 2023
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	$ 3,976.0	$ 3,481.2
Fair Value, Recurring
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivatives	21.5	1.3
Total assets	3,103.2	3,027.1
Derivatives	1.7
Total liabilities	1.7
Fair Value, Recurring | Fixed maturities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	2,368.2	2,585.4
Fair Value, Recurring | Fixed maturities | Collateralized Loan Obligations
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	147.3	214.1
Fair Value, Recurring | Fixed maturities | U.S. Governments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	264.9	362.2
Fair Value, Recurring | Fixed maturities | Foreign Governments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	7.4	30.3
Fair Value, Recurring | Fixed maturities | Obligations of states and political subdivisions
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	85.2	94.4
Fair Value, Recurring | Fixed maturities | Corporate bonds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	1,244.7	1,213.1
Fair Value, Recurring | Fixed maturities | Commercial mortgage-backed securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	291.7	280.5
Fair Value, Recurring | Fixed maturities | Residential mortgage-backed securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	218.6	248.8
Fair Value, Recurring | Fixed maturities | Asset-backed securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	108.4	142.0
Fair Value, Recurring | Equity securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	85.3	10.7
Fair Value, Recurring | Other investments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	37.7	0.2
Fair Value, Recurring | Short-term investments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	590.5	429.5
Fair Value, Recurring | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivatives	0.0	0.0
Total assets	856.6	793.4
Derivatives	0.0
Total liabilities	0.0
Fair Value, Recurring | Level 1 | Fixed maturities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	262.7	360.1
Fair Value, Recurring | Level 1 | Fixed maturities | Collateralized Loan Obligations
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	0.0	0.0
Fair Value, Recurring | Level 1 | Fixed maturities | U.S. Governments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	262.7	360.1
Fair Value, Recurring | Level 1 | Fixed maturities | Foreign Governments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	0.0	0.0
Fair Value, Recurring | Level 1 | Fixed maturities | Obligations of states and political subdivisions
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	0.0	0.0
Fair Value, Recurring | Level 1 | Fixed maturities | Corporate bonds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	0.0	0.0
Fair Value, Recurring | Level 1 | Fixed maturities | Commercial mortgage-backed securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	0.0	0.0
Fair Value, Recurring | Level 1 | Fixed maturities | Residential mortgage-backed securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	0.0	0.0
Fair Value, Recurring | Level 1 | Fixed maturities | Asset-backed securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	0.0	0.0
Fair Value, Recurring | Level 1 | Equity securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	3.4	4.3
Fair Value, Recurring | Level 1 | Other investments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	0.0	0.0
Fair Value, Recurring | Level 1 | Short-term investments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	590.5	429.0
Fair Value, Recurring | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivatives	21.5	1.3
Total assets	2,090.6	2,176.9
Derivatives	1.7
Total liabilities	1.7
Fair Value, Recurring | Level 2 | Fixed maturities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	2,069.1	2,174.9
Fair Value, Recurring | Level 2 | Fixed maturities | Collateralized Loan Obligations
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	147.3	214.1
Fair Value, Recurring | Level 2 | Fixed maturities | U.S. Governments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	2.2	2.1
Fair Value, Recurring | Level 2 | Fixed maturities | Foreign Governments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	7.4	30.3
Fair Value, Recurring | Level 2 | Fixed maturities | Obligations of states and political subdivisions
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	85.2	94.4
Fair Value, Recurring | Level 2 | Fixed maturities | Corporate bonds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	1,222.0	1,180.5
Fair Value, Recurring | Level 2 | Fixed maturities | Commercial mortgage-backed securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	291.7	280.5
Fair Value, Recurring | Level 2 | Fixed maturities | Residential mortgage-backed securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	218.6	248.8
Fair Value, Recurring | Level 2 | Fixed maturities | Asset-backed securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	94.7	124.2
Fair Value, Recurring | Level 2 | Equity securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	0.0	0.0
Fair Value, Recurring | Level 2 | Other investments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	0.0	0.2
Fair Value, Recurring | Level 2 | Short-term investments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	0.0	0.5
Fair Value, Recurring | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivatives	0.0	0.0
Total assets	156.0	56.8
Derivatives	0.0
Total liabilities	0.0
Fair Value, Recurring | Level 3 | Fixed maturities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	36.4	50.4
Fair Value, Recurring | Level 3 | Fixed maturities | Collateralized Loan Obligations
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	0.0	0.0
Fair Value, Recurring | Level 3 | Fixed maturities | U.S. Governments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	0.0	0.0
Fair Value, Recurring | Level 3 | Fixed maturities | Foreign Governments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	0.0	0.0
Fair Value, Recurring | Level 3 | Fixed maturities | Obligations of states and political subdivisions
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	0.0	0.0
Fair Value, Recurring | Level 3 | Fixed maturities | Corporate bonds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	22.7	32.6
Fair Value, Recurring | Level 3 | Fixed maturities | Commercial mortgage-backed securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	0.0	0.0
Fair Value, Recurring | Level 3 | Fixed maturities | Residential mortgage-backed securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	0.0	0.0
Fair Value, Recurring | Level 3 | Fixed maturities | Asset-backed securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	13.7	17.8
Fair Value, Recurring | Level 3 | Equity securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	81.9	6.4
Fair Value, Recurring | Level 3 | Other investments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	37.7	0.0
Fair Value, Recurring | Level 3 | Short-term investments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	$ 0.0	$ 0.0