Insurance Claim Reserves (Details) - Prior Year Reserve Development - Property-casualty [Member] - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Liability for claims and claim adjustment expense
Net favorable prior year reserve development included in estimated claims and claim adjustment expenses incurred for claims arising in prior years			$ 118	$ 268
Net favorable prior year reserve development impacting the Company's results of operations			174	336
Accretion expense			25	25
Business Insurance [Member]
Liability for claims and claim adjustment expense
Net favorable prior year reserve development impacting the Company's results of operations	$ 71	$ 84	50	150
Business Insurance [Member] | Environmental reserves [Member]
Liability for claims and claim adjustment expense
Increase to environmental reserves	60	55	60	55
Bond & Specialty Insurance [Member]
Liability for claims and claim adjustment expense
Net favorable prior year reserve development impacting the Company's results of operations	39	89	42	124
Personal Insurance [Member]
Liability for claims and claim adjustment expense
Net favorable prior year reserve development impacting the Company's results of operations	$ 13	$ 13	$ 82	$ 62