Insurance Claim Reserves (details) - Prior Year Reserve Development by Segment - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Liability for claims and claim adjustment expense
Net favorable (unfavorable) prior year reserve development impacting the Company's results of operations	$ 592	$ 771	$ 941
Business Insurance [Member]
Liability for claims and claim adjustment expense
Net favorable (unfavorable) prior year reserve development impacting the Company's results of operations	439	424	332
Bond & Specialty Insurance [Member]
Liability for claims and claim adjustment expense
Net favorable (unfavorable) prior year reserve development impacting the Company's results of operations	140	350	281
Personal Insurance [Member]
Liability for claims and claim adjustment expense
Net favorable (unfavorable) prior year reserve development impacting the Company's results of operations	$ 13	$ (3)	$ 328