Employee Benefit Plans (Net Periodic Benefit Costs) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Pension Benefits
Net periodic benefit costs [Abstract]
Service costs	$ 58	$ 62	$ 176	$ 184
Interest costs	95	106	287	318
Divestitures	0	3	0	3
Expected return on plan assets	(133)	(129)	(400)	(387)
Amortization of net actuarial (gains) losses	46	46	134	143
Amortization of prior service costs (credit)	(1)	0	(1)	(1)
Net periodic benefit costs (credit)	65	88	196	260
Other Postretirement Benefits
Net periodic benefit costs [Abstract]
Service costs	1	1	4	4
Interest costs	13	19	39	57
Divestitures	0	2	0	2
Expected return on plan assets	(18)	(18)	(54)	(54)
Amortization of net actuarial (gains) losses	(9)	0	(26)	0
Amortization of prior service costs (credit)	(5)	(6)	(14)	(17)
Net periodic benefit costs (credit)	$ (18)	$ (2)	$ (51)	$ (8)