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, 2023	Dec. 31, 2022
Schedule of Investments [Line Items]
Fair Value, Less Than One Year	$ 371.8	$ 1,557.5
Unrealized Losses, Less Than One Year	23.7	143.6
Fair Value, One Year or Greater	2,236.0	1,066.4
Unrealized Losses, One Year or Greater	281.7	196.6
Fair Value, Total	2,607.8	2,623.9
Unrealized Losses, Total	305.4	340.2
U.S. Governments
Schedule of Investments [Line Items]
Fair Value, Less Than One Year	17.3	271.0
Unrealized Losses, Less Than One Year	0.4	18.1
Fair Value, One Year or Greater	370.1	109.8
Unrealized Losses, One Year or Greater	24.2	12.1
Fair Value, Total	387.4	380.8
Unrealized Losses, Total	24.6	30.2
Foreign Governments
Schedule of Investments [Line Items]
Fair Value, Less Than One Year	13.1	16.7
Unrealized Losses, Less Than One Year	2.4	4.9
Fair Value, One Year or Greater	7.3	2.6
Unrealized Losses, One Year or Greater	4.8	1.8
Fair Value, Total	20.4	19.3
Unrealized Losses, Total	7.2	6.7
Obligations of states and political subdivisions
Schedule of Investments [Line Items]
Fair Value, Less Than One Year	23.3	67.4
Unrealized Losses, Less Than One Year	1.0	4.1
Fair Value, One Year or Greater	66.4	24.3
Unrealized Losses, One Year or Greater	7.3	6.0
Fair Value, Total	89.7	91.7
Unrealized Losses, Total	8.3	10.1
Corporate bonds
Schedule of Investments [Line Items]
Fair Value, Less Than One Year	169.9	695.1
Unrealized Losses, Less Than One Year	11.1	68.3
Fair Value, One Year or Greater	1,027.8	519.6
Unrealized Losses, One Year or Greater	129.6	91.7
Fair Value, Total	1,197.7	1,214.7
Unrealized Losses, Total	140.7	160.0
Commercial mortgage-backed securities
Schedule of Investments [Line Items]
Fair Value, Less Than One Year	29.1	144.2
Unrealized Losses, Less Than One Year	2.6	18.6
Fair Value, One Year or Greater	255.6	141.2
Unrealized Losses, One Year or Greater	47.8	33.4
Fair Value, Total	284.7	285.4
Unrealized Losses, Total	50.4	52.0
Residential mortgage-backed securities
Schedule of Investments [Line Items]
Fair Value, Less Than One Year	26.9	88.7
Unrealized Losses, Less Than One Year	1.5	8.8
Fair Value, One Year or Greater	238.7	178.8
Unrealized Losses, One Year or Greater	44.6	41.4
Fair Value, Total	265.6	267.5
Unrealized Losses, Total	46.1	50.2
Asset-backed securities
Schedule of Investments [Line Items]
Fair Value, Less Than One Year	43.8	93.3
Unrealized Losses, Less Than One Year	1.9	7.5
Fair Value, One Year or Greater	93.1	45.9
Unrealized Losses, One Year or Greater	10.2	6.7
Fair Value, Total	136.9	139.2
Unrealized Losses, Total	12.1	14.2
Collateralized loan obligations
Schedule of Investments [Line Items]
Fair Value, Less Than One Year	48.4	181.1
Unrealized Losses, Less Than One Year	2.8	13.3
Fair Value, One Year or Greater	177.0	44.2
Unrealized Losses, One Year or Greater	13.2	3.5
Fair Value, Total	225.4	225.3
Unrealized Losses, Total	$ 16.0	$ 16.8