Pension and Other Postretirement Benefit Plans - Obligation and Funded Status (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Pension Benefits, U.S.
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets January 1	$ 9,984	$ 10,007
Actual return on plan assets	(226)	484
Company contributions	66	92
Effects of exchange rate changes	0	0
Benefits paid	(523)	(535)
Settlements	(35)	(64)
Other	0	0
Fair value of plan assets December 31	9,266	9,984	$ 10,007
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation January 1	10,632	8,666
Service cost	307	300	386
Interest cost	434	425	402
Actuarial (gains) losses	(1,102)	1,857
Benefits paid	(523)	(535)
Effects of exchange rate changes	0	0
Plan amendments	0	0
Curtailments	(14)	(70)
Termination benefits	22	53	51
Settlements	(35)	(64)
Other	2	0
Benefit obligation December 31	9,723	10,632	8,666
Funded status December 31	(457)	(648)
Recognized as:
Other assets	179	68
Accrued and other current liabilities	(48)	(41)
Other noncurrent liabilities	(588)	(675)
Pension Benefits, International
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets January 1	7,724	7,428
Actual return on plan assets	138	1,099
Company contributions	163	276
Effects of exchange rate changes	(568)	(816)
Benefits paid	(196)	(245)
Settlements	(66)	(31)
Other	9	13
Fair value of plan assets December 31	7,204	7,724	7,428
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation January 1	8,331	7,389
Service cost	251	266	296
Interest cost	206	269	263
Actuarial (gains) losses	(127)	1,605
Benefits paid	(196)	(245)
Effects of exchange rate changes	(647)	(864)
Plan amendments	(1)	(4)
Curtailments	(15)	(76)
Termination benefits	1	11	7
Settlements	(66)	(31)
Other	(4)	11
Benefit obligation December 31	7,733	8,331	7,389
Funded status December 31	(529)	(607)
Recognized as:
Other assets	567	565
Accrued and other current liabilities	(7)	(11)
Other noncurrent liabilities	(1,089)	(1,161)
Other Postretirement Benefits
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets January 1	1,984	1,913
Actual return on plan assets	(34)	114
Company contributions	63	67
Effects of exchange rate changes	(1)	0
Benefits paid	(99)	(110)
Settlements	0	0
Other	0	0
Fair value of plan assets December 31	1,913	1,984	1,913
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation January 1	2,638	2,329
Service cost	80	78	102
Interest cost	110	115	107
Actuarial (gains) losses	(384)	212
Benefits paid	(99)	(110)
Effects of exchange rate changes	(11)	(6)
Plan amendments	(531)	0
Curtailments	(3)	3
Termination benefits	7	22	50
Settlements	0	0
Other	3	(5)
Benefit obligation December 31	1,810	2,638	$ 2,329
Funded status December 31	103	(654)
Recognized as:
Other assets	359	1
Accrued and other current liabilities	(10)	(11)
Other noncurrent liabilities	$ (246)	$ (644)