Investments - Financial Assets Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	$ 5,322.6	$ 5,255.8
Other assets
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	387.2	429.4
Recurring fair value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	5,010.8	4,928.9
Recurring fair value | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	1,112.9	1,069.6
Recurring fair value | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	3,880.4	3,834.8
Recurring fair value | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	17.5	24.5
Recurring fair value | Fixed maturities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	4,223.3	4,107.1
Recurring fair value | Fixed maturities | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	417.4	383.5
Recurring fair value | Fixed maturities | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	3,803.1	3,716.6
Recurring fair value | Fixed maturities | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	2.8	7.0
Recurring fair value | Fixed maturities | Collateralized loan obligations
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	336.1	289.6
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	336.1	289.6
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	425.0	399.8
Recurring fair value | Fixed maturities | U.S. Governments | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	417.4	383.5
Recurring fair value | Fixed maturities | U.S. Governments | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	7.6	16.3
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	232.8	294.8
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	232.8	294.8
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	171.3	170.4
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	171.3	170.4
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	1,983.3	1,981.8
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	1,980.5	1,974.8
Recurring fair value | Fixed maturities | Corporate bonds | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	2.8	7.0
Recurring fair value | Fixed maturities | Commercial mortgage-backed securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	418.7	339.7
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	418.7	339.7
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	482.5	508.2
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	482.5	508.2
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	173.6	122.8
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	173.6	122.8
Recurring fair value | Fixed maturities | Asset backed securities | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	0.0	0.0
Recurring fair value | Equity securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	56.3	176.7
Recurring fair value | Equity securities | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	41.6	159.2
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	14.7	17.5
Recurring fair value | Other assets
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	75.4	102.5
Recurring fair value | Other assets | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	0.0	0.4
Recurring fair value | Other assets | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	75.4	102.1
Recurring fair value | Other assets | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	0.0	0.0
Recurring fair value | Other investments, including short-terms
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	655.8	542.6
Recurring fair value | Other investments, including short-terms | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	653.9	526.5
Recurring fair value | Other investments, including short-terms | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	1.9	16.1
Recurring fair value | Other investments, including short-terms | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	$ 0.0	$ 0.0