Investments - Financial Assets Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	$ 3,481.2	$ 3,651.9
Recurring fair value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivatives	1.3	5.2
Total assets	3,027.1	3,174.5
Recurring fair value | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivatives	0.0	0.0
Total assets	793.4	856.4
Recurring fair value | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivatives	1.3	5.2
Total assets	2,176.9	2,261.9
Recurring fair value | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivatives	0.0	0.0
Total assets	56.8	56.2
Recurring fair value | Fixed maturities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	2,585.4	2,675.5
Recurring fair value | Fixed maturities | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	360.1	378.7
Recurring fair value | Fixed maturities | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	2,174.9	2,256.1
Recurring fair value | Fixed maturities | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	50.4	40.7
Recurring fair value | Fixed maturities | Collateralized loan obligations
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	214.1	237.9
Recurring fair value | Fixed maturities | Collateralized loan obligations | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	0.0	0.0
Recurring fair value | Fixed maturities | Collateralized loan obligations | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	214.1	237.9
Recurring fair value | Fixed maturities | Collateralized loan obligations | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	0.0	0.0
Recurring fair value | Fixed maturities | U.S. Governments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	362.2	380.7
Recurring fair value | Fixed maturities | U.S. Governments | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	360.1	378.7
Recurring fair value | Fixed maturities | U.S. Governments | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	2.1	2.0
Recurring fair value | Fixed maturities | U.S. Governments | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	0.0	0.0
Recurring fair value | Fixed maturities | Foreign Governments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	30.3	28.4
Recurring fair value | Fixed maturities | Foreign Governments | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	0.0	0.0
Recurring fair value | Fixed maturities | Foreign Governments | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	30.3	28.4
Recurring fair value | Fixed maturities | Foreign Governments | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	0.0	0.0
Recurring fair value | Fixed maturities | Obligations of states and political subdivisions
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	94.4	99.8
Recurring fair value | Fixed maturities | Obligations of states and political subdivisions | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	0.0	0.0
Recurring fair value | Fixed maturities | Obligations of states and political subdivisions | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	94.4	99.8
Recurring fair value | Fixed maturities | Obligations of states and political subdivisions | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	0.0	0.0
Recurring fair value | Fixed maturities | Corporate bonds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	1,213.1	1,234.1
Recurring fair value | Fixed maturities | Corporate bonds | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	0.0	0.0
Recurring fair value | Fixed maturities | Corporate bonds | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	1,180.5	1,212.1
Recurring fair value | Fixed maturities | Corporate bonds | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	32.6	22.0
Recurring fair value | Fixed maturities | Commercial mortgage-backed securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	280.5	285.4
Recurring fair value | Fixed maturities | Commercial mortgage-backed securities | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	0.0	0.0
Recurring fair value | Fixed maturities | Commercial mortgage-backed securities | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	280.5	285.4
Recurring fair value | Fixed maturities | Commercial mortgage-backed securities | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	0.0	0.0
Recurring fair value | Fixed maturities | Residential mortgage-backed securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	248.8	270.0
Recurring fair value | Fixed maturities | Residential mortgage-backed securities | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	0.0	0.0
Recurring fair value | Fixed maturities | Residential mortgage-backed securities | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	248.8	270.0
Recurring fair value | Fixed maturities | Residential mortgage-backed securities | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	0.0	0.0
Recurring fair value | Fixed maturities | Asset-backed securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	142.0	139.2
Recurring fair value | Fixed maturities | Asset-backed securities | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	0.0	0.0
Recurring fair value | Fixed maturities | Asset-backed securities | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	124.2	120.5
Recurring fair value | Fixed maturities | Asset-backed securities | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	17.8	18.7
Recurring fair value | Equity securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	10.7	43.9
Recurring fair value | Equity securities | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	4.3	28.4
Recurring fair value | Equity securities | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	0.0	0.0
Recurring fair value | Equity securities | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	6.4	15.5
Recurring fair value | Other investments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	0.2	0.3
Recurring fair value | Other investments | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	0.0	0.0
Recurring fair value | Other investments | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	0.2	0.3
Recurring fair value | Other investments | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	0.0	0.0
Recurring fair value | Short-term investments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	429.5	449.6
Recurring fair value | Short-term investments | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	429.0	449.3
Recurring fair value | Short-term investments | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	0.5	0.3
Recurring fair value | Short-term investments | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	$ 0.0	$ 0.0