Employee Postretirement Benefits (Components of Net Periodic Benefit Cost) (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Pension Plans
Service cost		$ 22	$ 21	$ 36
Interest cost		95	121	128
Expected return on plan assets	[1]	(134)	(144)	(166)
Recognized net actuarial loss		42	44	47
Settlements and curtailments		49	45	136
Other		(4)	(4)	(7)
Net periodic benefit cost		70	83	174
Other Postretirement Benefit Plans
Service cost		8	8	11
Interest cost		23	28	28
Expected return on plan assets	[1]	0	0	0
Recognized net actuarial loss		1	0	1
Settlements and curtailments		0	0	0
Other		(2)	(1)	(3)
Net periodic benefit cost		$ 30	$ 35	$ 37

[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.