Insurance Claim Reserves (Details) - Prior Year Reserve Development - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Liability for claims and claim adjustment expense
Net favorable prior year reserve development impacting the Company's results of operations			$ 336	$ 284
Business Insurance [Member]
Liability for claims and claim adjustment expense
Net favorable prior year reserve development impacting the Company's results of operations	$ 84	$ 125	150	186
Bond & Specialty Insurance [Member]
Liability for claims and claim adjustment expense
Net favorable prior year reserve development impacting the Company's results of operations	89	78	124	92
Personal Insurance [Member]
Liability for claims and claim adjustment expense
Net favorable prior year reserve development impacting the Company's results of operations	$ 13	$ 0	$ 62	$ 6