Loss and Loss Adjustment Expense Reserves (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020	Dec. 31, 2019
Liability for Claims and Claims Adjustment Expense [Line Items]
Loss and loss adjustment expense reserves	$ 21,063.7	$ 18,306.5	$ 20,265.8	$ 18,105.4
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments and Liability for Unpaid Claims and Claim Adjustment Expense, Other Reconciling Item	3,968.6	3,443.7	3,798.2	3,212.2
Liability for Unpaid Claims and Claims Adjustment Expense, Net	17,095.1	14,862.8	$ 16,467.6	$ 14,893.2
Current Year Claims and Claims Adjustment Expense	6,986.1	6,011.1
Prior Year Claims and Claims Adjustment Expense	124.4	144.1
Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims	7,110.5	6,155.2
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Current Year	3,029.3	2,916.4
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Prior Years	3,453.7	3,269.2
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid	$ 6,483.0	$ 6,185.6
Short-Duration Insurance Contract, Accident Year 2020
Liability for Claims and Claims Adjustment Expense [Line Items]
Percent of Prior Year Claims and Claims Adjustment Expense	34.00%
Short-Duration Insurance Contract, Accident Year 2019
Liability for Claims and Claims Adjustment Expense [Line Items]
Percent of Prior Year Claims and Claims Adjustment Expense	34.00%	52.00%
Short-duration Insurance Contracts, Accident Year 2018
Liability for Claims and Claims Adjustment Expense [Line Items]
Percent of Prior Year Claims and Claims Adjustment Expense		36.00%
Personal Lines | Personal auto businesses
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense	$ 92.0	$ 104.0
Personal Lines | Special Lines
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense	(5.0)	(14.0)
Commercial Lines
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense	29.0	$ 54.0
Property
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense	$ 8.0
Agency | Personal Lines | Personal auto businesses
Liability for Claims and Claims Adjustment Expense [Line Items]
Percent of Prior Year Claims and Claims Adjustment Expense	60.00%