Insurance Claim Reserves (details) - Asbestos and Environmental Reserves - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Asbestos reserves [Member]
Liability for claims and claim adjustment expense
Increase (decrease) to asbestos and environmental reserves	$ 225	$ 225	$ 224
Net losses paid	$ 271	$ 708	$ 770
Percent of total asbestos net paid losses related to policyholders with whom the Company had entered into settlement agreements limiting the Company's liability	4.00%	69.00%	69.00%
Asbestos reserves [Member] | Asbestos Direct Action Litigation [Member]
Liability for claims and claim adjustment expense
Settlement amount paid			$ 502
Asbestos reserves [Member] | PPG Industries, Inc Litigation [Member]
Liability for claims and claim adjustment expense
Settlement amount paid		$ 458
Environmental reserves [Member]
Liability for claims and claim adjustment expense
Increase (decrease) to asbestos and environmental reserves	$ 65	82	72
Business Insurance [Member] | Asbestos reserves [Member]
Liability for claims and claim adjustment expense
Increase (decrease) to asbestos and environmental reserves	225	225	224
Business Insurance [Member] | Environmental reserves [Member]
Liability for claims and claim adjustment expense
Increase (decrease) to asbestos and environmental reserves	$ 65	$ 82	$ 72