Losses, Claims and Settlement Expenses - Consolidated - Reconciliation of Claims Development to Liability (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Net claim and allocated loss adjustment expense reserves		$ 6,651.9	$ 6,529.0
Reinsurance recoverable on claim reserves		4,973.6	4,695.3
Insurance coverages other than short-duration (b)	[1]	566.4	656.6
Unallocated loss adjustment expense reserves (c)	[2]	346.1	340.5
Insurance lines other than short-duration and unallocated loss adjustment expense reserves		912.6	997.1
Loss and loss adjustment expense reserves		12,538.2	12,221.5	$ 11,425.5	$ 10,671.0
Interest Accretion Amount		25.6	9.6	$ 42.0
Short-Duration Insurance Contract, Discounted Liability, Discount		179.9	184.7
Disposal Group, Not Discontinued Operations
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Insurance coverages other than short-duration (b)		53.6
Unallocated loss adjustment expense reserves (c)		1.2
Loss and loss adjustment expense reserves		54.9
Workers' Compensation [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Net claim and allocated loss adjustment expense reserves	[3]	2,725.3	2,879.6
Reinsurance recoverable on claim reserves		1,998.2	1,975.5
General Liability [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Net claim and allocated loss adjustment expense reserves		705.5	641.9
Reinsurance recoverable on claim reserves		813.2	785.3
Commercial Automobile [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Net claim and allocated loss adjustment expense reserves		1,808.4	1,747.3
Reinsurance recoverable on claim reserves		1,684.4	1,486.6
Other short-duration insurance lines [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Net claim and allocated loss adjustment expense reserves		1,412.5	1,260.0
Reinsurance recoverable on claim reserves		$ 477.7	$ 447.7

[1] RFIG Run-off loss reserves of $53.6 have been classified as held-for-sale as of December 31, 2023. See Note 2 for further discussion.

[2] RFIG Run-off unallocated loss adjustment expense reserves of $1.2 have been classified as held-for-sale as of December 31, 2023. See Note 2 for further discussion.

[3] Certain long-term disability type workers' compensation reserves are discounted to present value based on interest rates typically ranging from 3.0% to 3.5%. The amount of discount reflected in the year-end net reserves totaled $179.9 and $184.7 as of December 31, 2023 and 2022, respectively. Interest accretion of $25.6, $9.6, and $42.0 for the years ended December 31, 2023, 2022, and 2021, respectively, was recognized as unfavorable development of prior year reserves within loss and loss adjustment expenses in the consolidated statements of income.