Summary of Significant Accounting Policies Losses, Claims and Settlement Expenses - Consolidated - Reconciliation of Claims Development to Liability (Details) - USD ($) $ in Millions		Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Net claim and allocated loss adjustment expense reserves		$ 6,059.7	$ 5,766.7
Reinsurance recoverable on claim reserves		3,645.1	3,242.8
Insurance coverages other than short-duration		654.5	621.5
Unallocated loss adjustment expense reserves		311.6	298.3
Insurance lines other than short-duration and unallocated loss adjustment expense reserves		966.2	919.9
Losses, claims, and settlement expenses		10,671.0	9,929.5	$ 9,471.2	$ 9,237.6
Workers' compensation discount, amount		196.9	209.6	$ 216.5
Three above coverages combined [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Net claim and allocated loss adjustment expense reserves		5,277.2	5,093.1
Reinsurance recoverable on claim reserves		3,341.3	2,997.4
Workers' Compensation [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Net claim and allocated loss adjustment expense reserves	[1]	3,044.1	3,079.1
Reinsurance recoverable on claim reserves		1,885.0	1,808.5
General Liability [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Net claim and allocated loss adjustment expense reserves		641.5	610.9
Reinsurance recoverable on claim reserves		667.9	643.7
Commercial Automobile [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Net claim and allocated loss adjustment expense reserves		1,591.5	1,403.0
Reinsurance recoverable on claim reserves		788.3	545.1
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		782.4	673.5
Reinsurance recoverable on claim reserves		$ 303.8	$ 245.3

[1] (a)    The amount of discount reflected in the year-end net reserves totaled $196.9 and $209.6 as of December 31, 2020 and 2019, respectively.