SCHEDULE I, CONDENSED FINANCIAL INFORMATION OF PARENT - FOOTNOTES (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016		Dec. 31, 2015
Basis of Presentation
Rabbi trust investment gains	$ 9	$ 7		$ 6
Gain (loss) on settlement	(7)	(21)	[1]	10	[1]
New Accounting Standards
Net cash used in operating activities	3,625	2,311	[1]	2,898	[1]
Net cash used in investing activities	(4,700)	(4,835)	[1]	(2,868)	[1]
Other (expense) income, net	254	132		126
Debt Instruments [Abstract]
Current portion of long-term debt	(1,427)	(913)	[2]
Total long-term debt	16,445	14,429	[2]
Long-term Debt, Fiscal Year Maturity [Abstract]
2018	1,412
2019	1,419
2020	1,033
2021	1,346
2022	647
Thereafter	11,282
Parent Company [Member]
Basis of Presentation
Rabbi trust investment gains	56	23		3
New Accounting Standards
Net cash used in operating activities	89	(3)		95
Net cash used in investing activities	(23)	452		(505)
Operation and maintenance	(87)	(81)		(66)
Other (expense) income, net	107	(2)		7
Debt Instruments [Abstract]
Market value adjustments for interest rate swaps, net	(1)	(3)
Build-to-suit lease	138	137
Gross long-term debt	6,737	5,734
Current portion of long-term debt	(500)	(600)
Unamortized discount on long-term debt	(13)	(10)
Unamortized debt issuance costs	(26)	(24)
Total long-term debt	6,198	5,100
Long-term Debt, Fiscal Year Maturity [Abstract]
2018	500
2019	1,000
2020	900
2021	850
2022	500
Thereafter	2,850
Parent Company [Member] | 2.3% Notes April 1, 2017 [Member]
Debt Instruments [Abstract]
Gross long-term debt	$ 0	600
Stated percentage rate	2.30%
Parent Company [Member] | 6.15% Notes June 15, 2018 [Member]
Debt Instruments [Abstract]
Gross long-term debt	$ 500	500
Stated percentage rate	6.15%
Parent Company [Member] | 9.8% Notes February 15, 2019 [Member]
Debt Instruments [Abstract]
Gross long-term debt	$ 500	500
Stated percentage rate	9.80%
Parent Company [Member] | 1.625% Notes October 7, 2019 [Member]
Debt Instruments [Abstract]
Gross long-term debt	$ 500	500
Stated percentage rate	1.625%
Parent Company [Member] | 2.4% Notes March 15, 2020 [Member]
Debt Instruments [Abstract]
Gross long-term debt	$ 500	500
Stated percentage rate	2.40%
Parent Company [Member] | 2.85% Notes November 15, 2020 [Member]
Debt Instruments [Abstract]
Gross long-term debt	$ 400	400
Stated percentage rate	2.85%
Parent Company [Member] | Notes at variable rates (2.038% at December 31, 2017) March 15, 2021 [Member]
Debt Instruments [Abstract]
Gross long-term debt	$ 850	0
Effective rate percentage	2.038%
Parent Company [Member] | 2.875% Notes October 1, 2022 [Member]
Debt Instruments [Abstract]
Gross long-term debt	$ 500	500
Stated percentage rate	2.875%
Parent Company [Member] | 4.05% Notes December 1, 2023 [Member]
Debt Instruments [Abstract]
Gross long-term debt	$ 500	500
Stated percentage rate	4.05%
Parent Company [Member] | 3.55% Notes June 15, 2024 [Member]
Debt Instruments [Abstract]
Gross long-term debt	$ 500	500
Stated percentage rate	3.55%
Parent Company [Member] | 3.75% Notes November 15, 2025 [Member]
Debt Instruments [Abstract]
Gross long-term debt	$ 350	350
Stated percentage rate	3.75%
Parent Company [Member] | 3.25% Notes June 15, 2027 [Member]
Debt Instruments [Abstract]
Gross long-term debt	$ 750	0
Stated percentage rate	3.25%
Parent Company [Member] | 6% Notes October 15, 2039 [Member]
Debt Instruments [Abstract]
Gross long-term debt	$ 750	750
Stated percentage rate	6.00%
Parent Company [Member] | Accounting Standards Update 2016-15 [Member] | New Accounting Pronouncement, Early Adoption, Effect [Member]
New Accounting Standards
Net cash used in operating activities		(3)		95
Dividends received from subsidiaries		0		0
Net cash used in investing activities		452		(505)
Parent Company [Member] | Foreign exchange instruments [Member]
Basis of Presentation
Gain (loss) on settlement	$ 50	(28)
As previously reported [Member] | Accounting Standards Update 2017-07 [Member]
New Accounting Standards
Other (expense) income, net	254	132
As previously reported [Member] | Parent Company [Member] | Accounting Standards Update 2016-15 [Member] | New Accounting Pronouncement, Early Adoption, Effect [Member]
New Accounting Standards
Net cash used in operating activities		(178)		(255)
Dividends received from subsidiaries		175		350
Net cash used in investing activities		627		(155)
As previously reported [Member] | Parent Company [Member] | Accounting Standards Update 2017-07 [Member]
New Accounting Standards
Operation and maintenance	(87)	(81)
Other (expense) income, net	107	(2)
Recast [Member] | Accounting Standards Update 2017-07 [Member]
New Accounting Standards
Other (expense) income, net	233	138
Recast [Member] | Parent Company [Member] | Accounting Standards Update 2017-07 [Member]
New Accounting Standards
Operation and maintenance	(80)	(76)
Other (expense) income, net	$ 100	(7)
Effect of adoption [Member] | Parent Company [Member] | Accounting Standards Update 2016-15 [Member] | New Accounting Pronouncement, Early Adoption, Effect [Member]
New Accounting Standards
Net cash used in operating activities		175		350
Dividends received from subsidiaries		(175)		(350)
Net cash used in investing activities		$ (175)		$ (350)

[1] As adjusted for the retrospective adoption of ASU 2016-15 and ASU 2016-18, which we discuss in Note 2.

[2] Reflects reclassifications to conform to current year presentation, which we discuss in Note 1.