Loss and Loss Adjustment Expense Reserves (Details) - USD ($) $ in Millions			6 Months Ended
	Jan. 01, 2022	Jan. 01, 2021	Jun. 30, 2022	Jun. 30, 2021	Dec. 31, 2021	Dec. 31, 2020
Liability for Claims and Claims Adjustment Expense [Line Items]
Loss and loss adjustment expense reserves			$ 27,812.0	$ 23,895.6	$ 26,164.1	$ 20,265.8
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments and Liability for Unpaid Claims and Claim Adjustment Expense, Other Reconciling Item			4,684.6	4,464.9	4,733.6	3,798.2
Liability for Unpaid Claims and Claims Adjustment Expense, Net	$ 21,430.5	$ 17,200.1	23,127.4	19,430.7	$ 21,430.5	$ 16,467.6
Liability for Unpaid Claims and Claims Adjustment Expense, Business Acquisitions	$ 0.0	$ 732.5
Current Year Claims and Claims Adjustment Expense			18,134.6	15,319.9
Prior Year Claims and Claims Adjustment Expense			144.9	197.0
Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims			18,279.5	15,516.9
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Current Year			9,298.3	7,849.6
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Prior Years			7,284.3	5,436.7
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid			16,582.6	13,286.3
Short-Duration Insurance Contract, Accident Year 2021
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense			97.0
Short-Duration Insurance Contract, Accident Year 2020
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense			16.0
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
Short-Duration Insurance Contract, Accident Years 2018 and Prior
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense				114.0
Personal Lines | Personal auto business
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense			45.0	111.0
Personal Lines | special lines
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense				(7.0)
Commercial Lines
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense			$ 98.0	73.0
Property
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense				20.0
Agency | Personal Lines | Personal auto business
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense				$ 67.0