Employee Postretirement Benefits (Components of Net Periodic Benefit Cost) (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Pension Plans
Service cost		$ 36	$ 41	$ 42
Interest cost		128	129	148
Expected return on plan assets	[1]	(166)	(156)	(158)
Recognized net actuarial loss		47	57	52
Settlements and curtailments		136	7	1
Other		(7)	(9)	(9)
Net periodic benefit cost		174	69	76
Other Postretirement Benefit Plans
Service cost		11	12	11
Interest cost		28	32	33
Expected return on plan assets	[1]	0	0	0
Recognized net actuarial loss		1	1	0
Settlements and curtailments		0	0	0
Other		(3)	(2)	(1)
Net periodic benefit cost		$ 37	$ 43	$ 43

[1] The expected return on plan assets is determined by multiplying the fair value of plan assets at the remeasurement date, typically the prior year-end adjusted for estimated current year cash benefit payments and contributions, by the expected long-term rate of return.