Pensions Benefit Obligation in Excess of Assets (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan, Plan with Benefit Obligation in Excess of Plan Assets [Abstract]
Projected benefit obligation	$ 2,059	$ 1,907
Fair value of plan assets	1,527	1,276
Defined Benefit Plan, Pension Plan with Accumulated Benefit Obligation in Excess of Plan Assets [Abstract]
Accumulated benefit obligation	1,962	1,847
Fair value of plan assets	$ 1,495	$ 1,275