Investments - Schedule of Aging of Unrealized Losses on Company's Investments in Fixed Maturities, Equity Securities and Other Investments (Detail) - Fixed maturities - USD ($) $ in Millions	Sep. 30, 2022	Dec. 31, 2021
Schedule of Investments [Line Items]
Fair Value, Less Than One Year	$ 2,941.5	$ 1,967.8
Unrealized Losses, Less Than One Year	246.7	36.9
Fair Value, One Year or Greater	788.8	70.0
Unrealized Losses, One Year or Greater	153.7	3.9
Fair Value, Total	3,730.3	2,037.8
Unrealized Losses, Total	400.4	40.8
U.S. Governments
Schedule of Investments [Line Items]
Fair Value, Less Than One Year	430.0	193.4
Unrealized Losses, Less Than One Year	26.0	2.6
Fair Value, One Year or Greater	81.8	14.6
Unrealized Losses, One Year or Greater	11.1	0.6
Fair Value, Total	511.8	208.0
Unrealized Losses, Total	37.1	3.2
Foreign Governments
Schedule of Investments [Line Items]
Fair Value, Less Than One Year	134.6	152.4
Unrealized Losses, Less Than One Year	17.4	3.3
Fair Value, One Year or Greater	63.7	2.6
Unrealized Losses, One Year or Greater	6.4	0.6
Fair Value, Total	198.3	155.0
Unrealized Losses, Total	23.8	3.9
Obligations of states and political subdivisions
Schedule of Investments [Line Items]
Fair Value, Less Than One Year	141.7	46.0
Unrealized Losses, Less Than One Year	6.4	0.8
Fair Value, One Year or Greater	19.3	0.1
Unrealized Losses, One Year or Greater	4.5	0.4
Fair Value, Total	161.0	46.1
Unrealized Losses, Total	10.9	1.2
Corporate bonds
Schedule of Investments [Line Items]
Fair Value, Less Than One Year	1,305.7	854.3
Unrealized Losses, Less Than One Year	111.1	18.3
Fair Value, One Year or Greater	371.2	41.7
Unrealized Losses, One Year or Greater	75.8	2.0
Fair Value, Total	1,676.9	896.0
Unrealized Losses, Total	186.9	20.3
Commercial mortgage-backed securities
Schedule of Investments [Line Items]
Fair Value, Less Than One Year	243.2	198.8
Unrealized Losses, Less Than One Year	26.0	4.1
Fair Value, One Year or Greater	102.6	6.5
Unrealized Losses, One Year or Greater	26.6	0.2
Fair Value, Total	345.8	205.3
Unrealized Losses, Total	52.6	4.3
Residential mortgage-backed securities
Schedule of Investments [Line Items]
Fair Value, Less Than One Year	232.3	284.2
Unrealized Losses, Less Than One Year	25.1	5.6
Fair Value, One Year or Greater	130.6	4.0
Unrealized Losses, One Year or Greater	27.5	0.1
Fair Value, Total	362.9	288.2
Unrealized Losses, Total	52.6	5.7
Asset-backed securities
Schedule of Investments [Line Items]
Fair Value, Less Than One Year	175.8	62.6
Unrealized Losses, Less Than One Year	12.4	0.6
Fair Value, One Year or Greater	6.6	0.0
Unrealized Losses, One Year or Greater	0.9	0.0
Fair Value, Total	182.4	62.6
Unrealized Losses, Total	13.3	0.6
Collateralized loan obligations
Schedule of Investments [Line Items]
Fair Value, Less Than One Year	278.2	176.1
Unrealized Losses, Less Than One Year	22.3	1.6
Fair Value, One Year or Greater	13.0	0.5
Unrealized Losses, One Year or Greater	0.9	0.0
Fair Value, Total	291.2	176.6
Unrealized Losses, Total	$ 23.2	$ 1.6