Investments - Financial Assets Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Millions	Mar. 31, 2024	Dec. 31, 2023
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	$ 4,011.1	$ 3,481.2
Fair Value, Recurring
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivatives	0.3	1.3
Total assets	3,497.2	3,027.1
Fair Value, Recurring | Fixed maturities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	2,413.4	2,585.4
Fair Value, Recurring | Fixed maturities | Collateralized Loan Obligations
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	201.1	214.1
Fair Value, Recurring | Fixed maturities | U.S. Governments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	356.0	362.2
Fair Value, Recurring | Fixed maturities | Foreign Governments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	6.2	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	88.7	94.4
Fair Value, Recurring | Fixed maturities | Corporate bonds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	1,115.5	1,213.1
Fair Value, Recurring | Fixed maturities | Commercial mortgage-backed securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	275.4	280.5
Fair Value, Recurring | Fixed maturities | Residential mortgage-backed securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	236.8	248.8
Fair Value, Recurring | Fixed maturities | Asset-backed securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	133.7	142.0
Fair Value, Recurring | Equity securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	8.6	10.7
Fair Value, Recurring | Other investments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	1.3	0.2
Fair Value, Recurring | Short-term investments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	1,073.6	429.5
Fair Value, Recurring | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivatives	0.0	0.0
Total assets	1,169.0	793.4
Fair Value, Recurring | Level 1 | Fixed maturities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	93.8	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	93.8	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	2.0	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	1,073.2	429.0
Fair Value, Recurring | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivatives	0.3	1.3
Total assets	2,285.5	2,176.9
Fair Value, Recurring | Level 2 | Fixed maturities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	2,284.6	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	201.1	214.1
Fair Value, Recurring | Level 2 | Fixed maturities | U.S. Governments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	262.2	2.1
Fair Value, Recurring | Level 2 | Fixed maturities | Foreign Governments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	6.2	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	88.7	94.4
Fair Value, Recurring | Level 2 | Fixed maturities | Corporate bonds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	1,093.2	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	275.4	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	236.8	248.8
Fair Value, Recurring | Level 2 | Fixed maturities | Asset-backed securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	121.0	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	0.2	0.2
Fair Value, Recurring | Level 2 | Short-term investments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	0.4	0.5
Fair Value, Recurring | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivatives	0.0	0.0
Total assets	42.7	56.8
Fair Value, Recurring | Level 3 | Fixed maturities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	35.0	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	22.3	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	12.7	17.8
Fair Value, Recurring | Level 3 | Equity securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	6.6	6.4
Fair Value, Recurring | Level 3 | Other investments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	1.1	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