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, 2016 |
Sep. 30, 2016 |
Jun. 30, 2016 |
Mar. 31, 2016 |
Dec. 31, 2015 |
Sep. 30, 2015 |
Jun. 30, 2015 |
Mar. 31, 2015 |
Dec. 31, 2016 |
Dec. 31, 2015 |
Dec. 31, 2014 |
|
| Condensed Income Statements, Captions [Line Items] | |||||||||||
| Interest expense | $ (553) | $ (561) | $ (554) | ||||||||
| Other (expense) income, net | 132 | 126 | 137 | ||||||||
| Income tax (expense) benefit | (389) | (341) | (300) | ||||||||
| Equity in earnings of subsidiaries, net of income taxes | 78 | 85 | 38 | ||||||||
| Net income/Earnings | $ 379 | $ 622 | $ 16 | $ 353 | $ 369 | $ 248 | $ 295 | $ 437 | $ 1,370 | $ 1,349 | $ 1,161 |
| Basic earnings per common share (in dollars per share) | $ 1.51 | $ 2.48 | $ 0.06 | $ 1.41 | $ 1.48 | $ 1.00 | $ 1.19 | $ 1.76 | $ 5.48 | $ 5.43 | $ 4.72 |
| Weighted-average number of shares outstanding for basic EPS | 250,600 | 250,400 | 250,100 | 249,700 | 248,700 | 248,400 | 248,100 | 247,700 | 250,217 | 248,249 | 245,891 |
| Earnings Per Share, Diluted | $ 1.51 | $ 2.46 | $ 0.06 | $ 1.40 | $ 1.47 | $ 0.99 | $ 1.17 | $ 1.74 | $ 5.46 | $ 5.37 | $ 4.63 |
| Weighted-average common shares outstanding for diluted EPS | 251,600 | 252,400 | 252,000 | 251,500 | 251,500 | 251,000 | 251,500 | 251,200 | 251,155 | 250,923 | 250,655 |
| Parent Company [Member] | |||||||||||
| Condensed Income Statements, Captions [Line Items] | |||||||||||
| Interest expense | $ (277) | $ (261) | $ (235) | ||||||||
| Operation and maintenance | (81) | (66) | (78) | ||||||||
| Other (expense) income, net | (2) | 7 | 50 | ||||||||
| Income tax (expense) benefit | 181 | 150 | 133 | ||||||||
| Loss before equity in earnings of subsidiaries | (179) | (170) | (130) | ||||||||
| Equity in earnings of subsidiaries, net of income taxes | 1,549 | 1,519 | 1,291 | ||||||||
| Net income/Earnings | $ 1,370 | $ 1,349 | $ 1,161 | ||||||||
| Basic earnings per common share (in dollars per share) | $ 5.48 | $ 5.43 | $ 4.72 | ||||||||
| Weighted-average number of shares outstanding for basic EPS | 250,217 | 248,249 | 245,891 | ||||||||
| Earnings Per Share, Diluted | $ 5.46 | $ 5.37 | $ 4.63 | ||||||||
| Weighted-average common shares outstanding for diluted EPS | 251,155 | 250,923 | 250,655 | ||||||||