Insurance Claim Reserves (details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Insurance Claim Reserves disclosure
Amount of increase (decrease) in gross claims and claim adjustment expense reserves	$ 1,700	$ (343)
Amount of increase (decrease) in reinsurance recoverables on unpaid losses	$ 142	$ (468)
Reserves for long-term disability and annuity claim payments primarily arising from workers' compensation insurance and workers' compensation excess insurance policies, discount rate (percent)	5.00%	5.00%
Reserves for long-term disability and annuity claim payments primarily arising from workers' compensation insurance and workers' compensation excess insurance policies, net of discount	$ 2,320	$ 2,170
Discount included in reserves for long-term disability and annuity claim payments primarily arising from workers' compensation insurance and workers' compensation excess insurance policies	1,102	1,083
Accretion of discount	50	50	$ 51
Asbestos and environmental claims reserves, balance	$ 1,640	$ 1,710