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	Jun. 30, 2022	Dec. 31, 2021
Schedule of Investments [Line Items]
Fair Value, Less Than One Year	$ 3,183.4	$ 1,967.8
Unrealized Losses, Less Than One Year	239.8	36.9
Fair Value, One Year or Greater	462.4	70.0
Unrealized Losses, One Year or Greater	70.7	3.9
Fair Value, Total	3,645.8	2,037.8
Unrealized Losses, Total	310.5	40.8
U.S. Governments
Schedule of Investments [Line Items]
Fair Value, Less Than One Year	429.0	193.4
Unrealized Losses, Less Than One Year	18.3	2.6
Fair Value, One Year or Greater	48.7	14.6
Unrealized Losses, One Year or Greater	4.9	0.6
Fair Value, Total	477.7	208.0
Unrealized Losses, Total	23.2	3.2
Foreign Governments
Schedule of Investments [Line Items]
Fair Value, Less Than One Year	165.2	152.4
Unrealized Losses, Less Than One Year	14.1	3.3
Fair Value, One Year or Greater	27.4	2.6
Unrealized Losses, One Year or Greater	1.1	0.6
Fair Value, Total	192.6	155.0
Unrealized Losses, Total	15.2	3.9
Obligations of states and political subdivisions
Schedule of Investments [Line Items]
Fair Value, Less Than One Year	109.7	46.0
Unrealized Losses, Less Than One Year	7.5	0.8
Fair Value, One Year or Greater	3.2	0.1
Unrealized Losses, One Year or Greater	0.3	0.4
Fair Value, Total	112.9	46.1
Unrealized Losses, Total	7.8	1.2
Corporate bonds
Schedule of Investments [Line Items]
Fair Value, Less Than One Year	1,443.7	854.3
Unrealized Losses, Less Than One Year	116.4	18.3
Fair Value, One Year or Greater	216.9	41.7
Unrealized Losses, One Year or Greater	36.2	2.0
Fair Value, Total	1,660.6	896.0
Unrealized Losses, Total	152.6	20.3
Commercial mortgage-backed securities
Schedule of Investments [Line Items]
Fair Value, Less Than One Year	327.8	198.8
Unrealized Losses, Less Than One Year	32.5	4.1
Fair Value, One Year or Greater	39.3	6.5
Unrealized Losses, One Year or Greater	8.1	0.2
Fair Value, Total	367.1	205.3
Unrealized Losses, Total	40.6	4.3
Residential mortgage-backed securities
Schedule of Investments [Line Items]
Fair Value, Less Than One Year	260.2	284.2
Unrealized Losses, Less Than One Year	24.6	5.6
Fair Value, One Year or Greater	109.3	4.0
Unrealized Losses, One Year or Greater	19.0	0.1
Fair Value, Total	369.5	288.2
Unrealized Losses, Total	43.6	5.7
Asset-backed securities
Schedule of Investments [Line Items]
Fair Value, Less Than One Year	167.9	62.6
Unrealized Losses, Less Than One Year	9.9	0.6
Fair Value, One Year or Greater	6.8	0.0
Unrealized Losses, One Year or Greater	0.7	0.0
Fair Value, Total	174.7	62.6
Unrealized Losses, Total	10.6	0.6
Collateralized loan obligations
Schedule of Investments [Line Items]
Fair Value, Less Than One Year	279.9	176.1
Unrealized Losses, Less Than One Year	16.5	1.6
Fair Value, One Year or Greater	10.8	0.5
Unrealized Losses, One Year or Greater	0.4	0.0
Fair Value, Total	290.7	176.6
Unrealized Losses, Total	$ 16.9	$ 1.6