SCHEDULE I, CONDENSED FINANCIAL INFORMATION OF PARENT - CASH FLOWS (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016		Dec. 31, 2015
Condensed Cash Flow Statements, Captions [Line Items]
Net cash used in operating activities		$ 3,625	$ 2,311	[1]	$ 2,898	[1]
Expenditures for property, plant and equipment		(3,949)	(4,214)	[1]	(3,156)	[1]
Purchase of trust assets		(2)	9	[1]	9	[1]
Net cash used in investing activities		(4,700)	(4,835)	[1]	(2,868)	[1]
Common stock dividends paid		(755)	(686)	[1]	(628)	[1]
Issuances of common stock		47	51	[1]	52	[1]
Repurchases of common stock		(15)	(56)	[1]	(74)	[1]
Issuances of long-term debt		4,509	2,951	[1]	2,992	[1]
Payments on long-term debt		(2,800)	(2,057)	[1]	(1,854)	[1]
Tax benefit related to share-based compensation		0	0	[1]	52	[1]
Other		(43)	(21)	[1]	(20)	[1]
Net cash provided by (used in) financing activities		1,007	2,502	[1]	(176)	[1]
Cash and cash equivalents, January 1	[2]	349
Cash and cash equivalents, December 31		288	349	[2]
SUPPLEMENTAL DISCLOSURE OF NONCASH FINANCING ACTIVITIES
Accrued Merger-related transaction costs		31	0	[1]	0	[1]
Financing of build-to-suit property		0	0	[1]	61	[1]
Common dividends issued in stock		53	53	[1]	55	[1]
Dividends declared but not paid		214	196	[1]	180	[1]
Parent Company [Member]
Condensed Cash Flow Statements, Captions [Line Items]
Net cash used in operating activities		89	(3)		95
Expenditures for property, plant and equipment		(11)	(5)	[1]	(43)	[1]
Purchase of trust assets		0	0		(5)
Decrease (increase) in loans to affiliate, net		0	457	[1]	(457)	[1]
Expenditures for Merger-related transaction costs		(12)	0		0
Net cash used in investing activities		(23)	452		(505)
Common stock dividends paid		(755)	(686)		(628)
Issuances of common stock		47	51		52
Repurchases of common stock		(15)	(56)		(74)
Issuances of long-term debt		1,595	499		1,248
Payments on long-term debt		(600)	(750)		0
Increase (decrease) in loans from affiliates, net		(239)	504		(230)
Tax benefit related to share-based compensation		0	0		52
Other		(7)	(3)		(9)
Net cash provided by (used in) financing activities		26	(441)		411
(Decrease) increase in cash and cash equivalents		92	8		1
Cash and cash equivalents, January 1		12	4		3
Cash and cash equivalents, December 31		104	12		4
SUPPLEMENTAL DISCLOSURE OF NONCASH FINANCING ACTIVITIES
Accrued Merger-related transaction costs		31	0		0
Financing of build-to-suit property		0	0		61
Common dividends issued in stock		53	53		55
Dividends declared but not paid		$ 207	$ 189		$ 174

[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.