Loss and Loss Adjustment Expense Reserves (Details) - USD ($) $ in Millions			6 Months Ended
	Jan. 01, 2021	Jan. 01, 2020	Jun. 30, 2021	Jun. 30, 2020	Dec. 31, 2020	Dec. 31, 2019
Liability for Claims and Claims Adjustment Expense [Line Items]
Loss and loss adjustment expense reserves			$ 23,895.6	$ 18,512.0	$ 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			4,464.9	3,502.7	3,798.2	3,212.2
Liability for Unpaid Claims and Claims Adjustment Expense, Net	$ 17,200.1	$ 14,893.2	19,430.7	15,009.3	$ 16,467.6	$ 14,893.2
Liability for Unpaid Claims and Claims Adjustment Expense, Business Acquisitions	$ 732.5	$ 0.0
Current Year Claims and Claims Adjustment Expense			15,319.9	11,360.5
Prior Year Claims and Claims Adjustment Expense			(197.0)	(116.1)
Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims			15,516.9	11,476.6
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Current Year			7,849.6	6,180.4
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Prior Years			5,436.7	5,180.1
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid			13,286.3	$ 11,360.5
Short-Duration Insurance Contract, Accident Year 2019
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense			(100.0)
Percent of Prior Year Claims and Claims Adjustment Expense				1.00%
Short-duration Insurance Contracts, Accident Year 2018
Liability for Claims and Claims Adjustment Expense [Line Items]
Previous Years Claims and Claims Adjustment Expense			114.0
Percent of Prior Year Claims and Claims Adjustment Expense				68.00%
Personal Lines | Personal auto businesses
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense			(111.0)	$ (37.0)
Personal Lines | Special Lines
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense			7.0	15.0
Commercial Lines
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense			(73.0)	(98.0)
Property
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense			$ (20.0)	$ 4.0
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%