Summary of Significant Accounting Policies Losses, Claims and Settlement Expenses - Consolidated - Reconciliation of Claims Development to Liability (Details) - USD ($) $ in Millions		Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Net claim and allocated loss adjustment expense reserves		$ 5,766.7	$ 5,535.6
Reinsurance recoverable on claim reserves		3,242.8	2,992.3
Insurance coverages other than short-duration		621.5	660.5
Unallocated loss adjustment expense reserves		298.3	282.6
Insurance lines other than short-duration and unallocated loss adjustment expense reserves		919.9	943.2
Losses, claims, and settlement expenses		9,929.5	9,471.2	$ 9,237.6	$ 9,206.0
Workers' compensation discount, amount		209.6	216.5	$ 240.7
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		5,093.1	4,911.1
Reinsurance recoverable on claim reserves		2,997.4	2,746.2
Workers' Compensation [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Net claim and allocated loss adjustment expense reserves	[1]	3,079.1	3,090.0
Reinsurance recoverable on claim reserves		1,808.5	1,774.3
General Liability [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Net claim and allocated loss adjustment expense reserves		610.9	556.1
Reinsurance recoverable on claim reserves		643.7	561.5
Commercial Automobile [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Net claim and allocated loss adjustment expense reserves		1,403.0	1,264.9
Reinsurance recoverable on claim reserves		545.1	410.2
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		673.5	624.5
Reinsurance recoverable on claim reserves		$ 245.3	$ 246.1

[1] (a)    The amount of discount reflected in the year-end net reserves totaled $209.6 and $216.5 as of December 31, 2019 and 2018, respectively.