Investments - Financial Assets Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Millions	Dec. 31, 2022	Dec. 31, 2021
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	$ 3,651.9	$ 5,322.6
Recurring fair value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivatives	5.2
Total assets	3,174.5	5,010.8
Recurring fair value | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivatives	0.0
Total assets	856.4	1,112.9
Recurring fair value | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivatives	5.2
Total assets	2,261.9	3,880.4
Recurring fair value | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivatives	0.0
Total assets	56.2	17.5
Recurring fair value | Fixed maturities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	2,675.5	4,223.3
Recurring fair value | Fixed maturities | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	378.7	417.4
Recurring fair value | Fixed maturities | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	2,256.1	3,803.1
Recurring fair value | Fixed maturities | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	40.7	2.8
Recurring fair value | Fixed maturities | Collateralized loan obligations
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	237.9	336.1
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	237.9	336.1
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	380.7	425.0
Recurring fair value | Fixed maturities | U.S. Governments | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	378.7	417.4
Recurring fair value | Fixed maturities | U.S. Governments | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	2.0	7.6
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	28.4	232.8
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	28.4	232.8
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	99.8	171.3
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	99.8	171.3
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	1,234.1	1,983.3
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	1,212.1	1,980.5
Recurring fair value | Fixed maturities | Corporate bonds | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	22.0	2.8
Recurring fair value | Fixed maturities | Commercial mortgage-backed securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	285.4	418.7
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	285.4	418.7
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	270.0	482.5
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	270.0	482.5
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	139.2	173.6
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	120.5	173.6
Recurring fair value | Fixed maturities | Asset-backed securities | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	18.7	0.0
Recurring fair value | Equity securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	43.9	56.3
Recurring fair value | Equity securities | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	28.4	41.6
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	15.5	14.7
Recurring fair value | Other investments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	0.3	75.4
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	0.3	75.4
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	449.6	655.8
Recurring fair value | Short-term investments | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	449.3	653.9
Recurring fair value | Short-term investments | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	0.3	1.9
Recurring fair value | Short-term investments | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	$ 0.0	$ 0.0