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	Mar. 31, 2022	Dec. 31, 2021
Schedule of Investments [Line Items]
Fair Value, Less Than One Year	$ 2,862.1	$ 1,967.8
Unrealized Losses, Less Than One Year	128.2	36.9
Fair Value, One Year or Greater	468.0	70.0
Unrealized Losses, One Year or Greater	47.2	3.9
Fair Value, Total	3,330.1	2,037.8
Unrealized Losses, Total	175.4	40.8
U.S. Governments
Schedule of Investments [Line Items]
Fair Value, Less Than One Year	416.1	193.4
Unrealized Losses, Less Than One Year	12.8	2.6
Fair Value, One Year or Greater	49.6	14.6
Unrealized Losses, One Year or Greater	4.2	0.6
Fair Value, Total	465.7	208.0
Unrealized Losses, Total	17.0	3.2
Foreign Governments
Schedule of Investments [Line Items]
Fair Value, Less Than One Year	150.2	152.4
Unrealized Losses, Less Than One Year	6.7	3.3
Fair Value, One Year or Greater	25.4	2.6
Unrealized Losses, One Year or Greater	0.9	0.6
Fair Value, Total	175.6	155.0
Unrealized Losses, Total	7.6	3.9
Obligations of states and political subdivisions
Schedule of Investments [Line Items]
Fair Value, Less Than One Year	78.7	46.0
Unrealized Losses, Less Than One Year	4.7	0.8
Fair Value, One Year or Greater	3.0	0.1
Unrealized Losses, One Year or Greater	0.2	0.4
Fair Value, Total	81.7	46.1
Unrealized Losses, Total	4.9	1.2
Corporate bonds
Schedule of Investments [Line Items]
Fair Value, Less Than One Year	1,275.0	854.3
Unrealized Losses, Less Than One Year	62.9	18.3
Fair Value, One Year or Greater	219.6	41.7
Unrealized Losses, One Year or Greater	23.3	2.0
Fair Value, Total	1,494.6	896.0
Unrealized Losses, Total	86.2	20.3
Commercial mortgage-backed securities
Schedule of Investments [Line Items]
Fair Value, Less Than One Year	296.8	198.8
Unrealized Losses, Less Than One Year	17.8	4.1
Fair Value, One Year or Greater	41.7	6.5
Unrealized Losses, One Year or Greater	5.3	0.2
Fair Value, Total	338.5	205.3
Unrealized Losses, Total	23.1	4.3
Residential mortgage-backed securities
Schedule of Investments [Line Items]
Fair Value, Less Than One Year	234.6	284.2
Unrealized Losses, Less Than One Year	13.8	5.6
Fair Value, One Year or Greater	120.7	4.0
Unrealized Losses, One Year or Greater	12.7	0.1
Fair Value, Total	355.3	288.2
Unrealized Losses, Total	26.5	5.7
Asset-backed securities
Schedule of Investments [Line Items]
Fair Value, Less Than One Year	135.9	62.6
Unrealized Losses, Less Than One Year	4.7	0.6
Fair Value, One Year or Greater	7.0	0.0
Unrealized Losses, One Year or Greater	0.6	0.0
Fair Value, Total	142.9	62.6
Unrealized Losses, Total	5.3	0.6
Collateralized loan obligations
Schedule of Investments [Line Items]
Fair Value, Less Than One Year	274.8	176.1
Unrealized Losses, Less Than One Year	4.8	1.6
Fair Value, One Year or Greater	1.0	0.5
Unrealized Losses, One Year or Greater	0.0	0.0
Fair Value, Total	275.8	176.6
Unrealized Losses, Total	$ 4.8	$ 1.6