Income Taxes, Components of Income Tax Expense (Benefit) (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Sep. 30, 2016	Dec. 31, 2017		Dec. 31, 2016	Dec. 31, 2015
Current:
Country		$ 1,499		$ 488	$ 1,577
U.S. state / Canadian provincial		95		47	128
Total current		1,594		535	1,705
Deferred:
Country		(2,551)		238	151
U.S. state / Canadian provincial		8		(8)	14
Total deferred		(2,543)		230	165
Income tax expense (benefit)	$ (42)	(949)		765	1,870
Deferred income tax benefit included in tax reform		2,643
U.S. [Member]
Current:
Country		1,305		294	1,513
U.S. state / Canadian provincial		34		12	85
Total current		1,339	[1]	306	1,598
Deferred:
Country		(2,522)		203	143
U.S. state / Canadian provincial		23		(4)	(16)
Total deferred		(2,499)	[2]	199	127
Income tax expense (benefit)		(1,160)		505	1,725
Current income tax expense included in tax reform adjustment		781
Deferred income tax benefit included in tax reform		2,600
International [Member]
Current:
Country		194		194	64
U.S. state / Canadian provincial		61		35	43
Total current		255		229	107
Deferred:
Country		(29)		35	8
U.S. state / Canadian provincial		(15)		(4)	30
Total deferred		(44)		31	38
Income tax expense (benefit)		$ 211		$ 260	$ 145

[1] Current income tax expense includes the effect of our $781 million Tax Reform adjustment as described in “Tax Reform” above.

[2] Deferred income tax benefit includes the effect of our $2.6 billion Tax Reform adjustment as described in “Tax Reform” above.