Employee Postretirement Benefits (Components of Net Periodic Benefit Cost) (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Defined Benefit Pension Plans
Service cost		$ 21	$ 36	$ 41
Interest cost		121	128	129
Expected return on plan assets	[1]	(144)	(166)	(156)
Recognized net actuarial loss		44	47	57
Settlements and curtailments		45	136	7
Other		(4)	(7)	(9)
Net periodic benefit cost		83	174	69
Other Postretirement Benefit Plans
Service cost		8	11	12
Interest cost		28	28	32
Expected return on plan assets	[1]	0	0	0
Recognized net actuarial loss		0	1	1
Settlements and curtailments		0	0	0
Other		(1)	(3)	(2)
Net periodic benefit cost		$ 35	$ 37	$ 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.