Loss and Loss Adjustment Expense Reserves (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
Liability for Claims and Claims Adjustment Expense [Line Items]
Loss and loss adjustment expense reserves	$ 14,070.8	$ 12,060.4	$ 13,086.9	$ 11,368.0
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments and Liability for Unpaid Claims and Claim Adjustment Expense, Other Reconciling Item	2,293.1	1,947.9	2,170.1	1,801.0
Liability for Unpaid Claims and Claims Adjustment Expense, Net	11,777.7	10,112.5	$ 10,916.8	$ 9,567.0
Current Year Claims and Claims Adjustment Expense	10,164.6	8,804.1
Prior Year Claims and Claims Adjustment Expense	81.5	74.2
Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims	10,246.1	8,878.3
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Current Year	5,533.7	4,951.0
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Prior Years	3,851.5	3,381.8
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid	9,385.2	8,332.8
Commercial And Personal Auto Businesses [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		102.0
Special Lines and Property Businesses [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		(28.0)
Accident years 2017 and 2016
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense	72.0
Accident year 2016
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		56.0
Personal Lines | Personal auto businesses
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense	57.0
Personal Lines | Agency Auto [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense	36.0	65.0
Personal Lines | Direct Auto [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense	21.0	29.0
Property
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense	7.0	$ (20.0)
Commercial Lines
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense	$ 17.0