Commitments, Guarantees and Contingent Liabilities - New Jersey and North Carolina (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Dec. 31, 2016	Dec. 31, 2015
Loss Contingencies
Property and casualty insurance claims	$ 21,929.0		$ 22,221.0	$ 21,034.0
Earned premiums	35,115.0		$ 34,015.0	$ 32,915.0
North Carolina Reinsurance Facility
Loss Contingencies
Reinsurance recoverables on paid claims	12.5
Reinsurance recoverables on unpaid claims	73.8
Fund's surplus to cover future losses		$ 310.0
Property and casualty insurance claims		50.0
Earned premiums		1,000.0
Clean risk recoupment		173.0
Member loss recoupments		$ 149.0
North Carolina Joint Underwriters Association
Loss Contingencies
Plan surplus receivable	4.6
Funds surplus in member's equity	37.8
North Carolina Insurance Underwriting Association
Loss Contingencies
Funds surplus in member's equity	1,500.0
Maximum possible assessment	$ 1,000.0
Percentage of surcharge on property insurance policy statewide to be remitted to the plan	10.00%