SCHEDULE I, CONDENSED FINANCIAL INFORMATION OF PARENT - STATEMENT OF OPERATIONS (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Condensed Income Statements, Captions [Line Items]
Interest expense									$ (659)	$ (553)	$ (561)
Other (expense) income, net									254	132	126
Income tax (expense) benefit									(1,276)	(389)	(341)
Equity in earnings of subsidiaries, net of income taxes									42	78	85
Net income/Earnings	$ (501)	$ 57	$ 259	$ 441	$ 379	$ 622	$ 16	$ 353	$ 256	$ 1,370	$ 1,349
Basic earnings per common share (in dollars per share)	$ (1.99)	$ 0.23	$ 1.03	$ 1.76	$ 1.51	$ 2.48	$ 0.06	$ 1.41	$ 1.02	$ 5.48	$ 5.43
Weighted-average number of shares outstanding for basic EPS	251,900	251,700	251,400	251,100	250,600	250,400	250,100	249,700	251,545	250,217	248,249
Earnings Per Share, Diluted	$ (1.99)	$ 0.22	$ 1.03	$ 1.75	$ 1.51	$ 2.46	$ 0.06	$ 1.40	$ 1.01	$ 5.46	$ 5.37
Weighted-average common shares outstanding for diluted EPS	251,900	253,400	252,800	252,200	251,600	252,400	252,000	251,500	252,300	251,155	250,923
Parent Company [Member]
Condensed Income Statements, Captions [Line Items]
Interest expense									$ (293)	$ (277)	$ (261)
Operation and maintenance									(87)	(81)	(66)
Other (expense) income, net									107	(2)	7
Income tax (expense) benefit									33	181	150
Loss before equity in earnings of subsidiaries									(240)	(179)	(170)
Equity in earnings of subsidiaries, net of income taxes									496	1,549	1,519
Net income/Earnings									$ 256	$ 1,370	$ 1,349
Basic earnings per common share (in dollars per share)									$ 1.02	$ 5.48	$ 5.43
Weighted-average number of shares outstanding for basic EPS									251,545	250,217	248,249
Earnings Per Share, Diluted									$ 1.01	$ 5.46	$ 5.37
Weighted-average common shares outstanding for diluted EPS									252,300	251,155	250,923