Postretirement Benefit Plans (Cost of Pension Plans) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Principal pension plans
Defined Benefit Plan Disclosure [Line Items]
Service cost for benefits earned	$ 289	$ 315
Prior service cost (credit) amortization	(73)	(76)
Expected return on plan assets	(849)	(834)
Interest cost on benefit obligation	717	734
Net actuarial loss amortization	(710)	(612)
Curtailment loss (gain)	43	0
Pension plans costs	983	903
Other pension plans
Defined Benefit Plan Disclosure [Line Items]
Service cost for benefits earned	151	113
Prior service cost (credit) amortization	1	1
Expected return on plan assets	(294)	(263)
Interest cost on benefit obligation	142	172
Net actuarial loss amortization	(103)	(64)
Pension plans costs	101	85
Principal retiree benefit plans
Defined Benefit Plan Disclosure [Line Items]
Service cost for benefits earned	26	25
Prior service cost (credit) amortization	43	41
Expected return on plan assets	(9)	(11)
Interest cost on benefit obligation	57	63
Net actuarial loss amortization	(21)	(13)
Curtailment loss (gain)	3	0
Pension plans costs	$ 13	$ 23