Investments - Financial Assets Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Millions	Mar. 31, 2022	Dec. 31, 2021
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	$ 5,071.5	$ 5,322.6
Fair Value, Recurring
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	4,703.0	5,010.8
Fair Value, Recurring | Fixed maturities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	4,143.9	4,223.3
Fair Value, Recurring | Fixed maturities | Collateralized Loan Obligations
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	328.5	336.1
Fair Value, Recurring | Fixed maturities | U.S. Governments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	479.5	425.0
Fair Value, Recurring | Fixed maturities | Foreign Governments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	207.5	232.8
Fair Value, Recurring | Fixed maturities | Obligations of states and political subdivisions
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	166.6	171.3
Fair Value, Recurring | Fixed maturities | Corporate bonds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	1,930.3	1,983.3
Fair Value, Recurring | Fixed maturities | Commercial mortgage-backed securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	400.2	418.7
Fair Value, Recurring | Fixed maturities | Residential mortgage-backed securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	430.8	482.5
Fair Value, Recurring | Fixed maturities | Asset-backed securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	200.5	173.6
Fair Value, Recurring | Equity securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	54.0	56.3
Fair Value, Recurring | Other investments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	84.0	75.4
Fair Value, Recurring | Short-term investments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	421.1	655.8
Fair Value, Recurring | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	928.5	1,112.9
Fair Value, Recurring | Level 1 | Fixed maturities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	472.2	417.4
Fair Value, Recurring | Level 1 | Fixed maturities | Collateralized Loan Obligations
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	0.0	0.0
Fair Value, Recurring | Level 1 | Fixed maturities | U.S. Governments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	472.2	417.4
Fair Value, Recurring | Level 1 | Fixed maturities | Foreign Governments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	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]
Fair value	0.0	0.0
Fair Value, Recurring | Level 1 | Fixed maturities | Corporate bonds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	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]
Fair value	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]
Fair value	0.0	0.0
Fair Value, Recurring | Level 1 | Fixed maturities | Asset-backed securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	0.0	0.0
Fair Value, Recurring | Level 1 | Equity securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	36.4	41.6
Fair Value, Recurring | Level 1 | Other investments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	0.0	0.0
Fair Value, Recurring | Level 1 | Short-term investments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	419.9	653.9
Fair Value, Recurring | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	3,720.7	3,880.4
Fair Value, Recurring | Level 2 | Fixed maturities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	3,635.5	3,803.1
Fair Value, Recurring | Level 2 | Fixed maturities | Collateralized Loan Obligations
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	328.5	336.1
Fair Value, Recurring | Level 2 | Fixed maturities | U.S. Governments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	7.3	7.6
Fair Value, Recurring | Level 2 | Fixed maturities | Foreign Governments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	207.5	232.8
Fair Value, Recurring | Level 2 | Fixed maturities | Obligations of states and political subdivisions
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	166.6	171.3
Fair Value, Recurring | Level 2 | Fixed maturities | Corporate bonds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	1,915.3	1,980.5
Fair Value, Recurring | Level 2 | Fixed maturities | Commercial mortgage-backed securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	400.2	418.7
Fair Value, Recurring | Level 2 | Fixed maturities | Residential mortgage-backed securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	430.8	482.5
Fair Value, Recurring | Level 2 | Fixed maturities | Asset-backed securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	179.3	173.6
Fair Value, Recurring | Level 2 | Equity securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	0.0	0.0
Fair Value, Recurring | Level 2 | Other investments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	84.0	75.4
Fair Value, Recurring | Level 2 | Short-term investments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	1.2	1.9
Fair Value, Recurring | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	53.8	17.5
Fair Value, Recurring | Level 3 | Fixed maturities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	36.2	2.8
Fair Value, Recurring | Level 3 | Fixed maturities | Collateralized Loan Obligations
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	0.0	0.0
Fair Value, Recurring | Level 3 | Fixed maturities | U.S. Governments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	0.0	0.0
Fair Value, Recurring | Level 3 | Fixed maturities | Foreign Governments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	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]
Fair value	0.0	0.0
Fair Value, Recurring | Level 3 | Fixed maturities | Corporate bonds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	15.0	2.8
Fair Value, Recurring | Level 3 | Fixed maturities | Commercial mortgage-backed securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	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]
Fair value	0.0	0.0
Fair Value, Recurring | Level 3 | Fixed maturities | Asset-backed securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	21.2	0.0
Fair Value, Recurring | Level 3 | Equity securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	17.6	14.7
Fair Value, Recurring | Level 3 | Other investments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	0.0	0.0
Fair Value, Recurring | Level 3 | Short-term investments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	$ 0.0	$ 0.0