Employee Postretirement Benefits (Components of Net Periodic Benefit Cost) (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Defined Benefit Pension Plans
Service cost		$ 42	$ 38	$ 46
Interest cost		148	187	279
Expected return on plan assets(a)	[1]	(158)	(215)	(332)
Recognized net actuarial loss		52	75	100
Settlements		1	1,357	20
Other		(9)	(10)	(3)
Net periodic benefit cost		76	1,432	110
Other Postretirement Benefit Plans
Service cost		11	12	13
Interest cost		33	32	35
Expected return on plan assets(a)		0	0	0
Recognized net actuarial loss		0	0	0
Settlements		0	0	0
Other		(1)	(1)	(1)
Net periodic benefit cost		$ 43	$ 43	$ 47

[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.