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SCHEDULE I, CONDENSED FINANCIAL INFORMATION OF PARENT, CASH FLOWS (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Condensed Cash Flow Statements, Captions [Line Items]      
Net cash used in operating activities $ 2,319 $ 2,905 $ 2,161
Expenditures for property, plant and equipment (4,214) (3,156) (3,123)
Purchase of trust assets 0 1 35
Decrease (increase) in loans to affiliates, net 11 74 18
Net cash used in investing activities (4,886) (2,885) (3,342)
Common stock dividends paid (686) (628) (598)
Issuances of common stock 51 52 56
Repurchases of common stock (56) (74) (38)
Issuances of long-term debt 2,951 2,992 3,272
Payments on long-term debt (2,057) (1,854) (2,034)
Tax benefit related to share-based compensation 0 52 0
Other (10) (17) (37)
Net cash provided by (used in) financing activities 2,513 (173) 854
Increase (decrease) in cash and cash equivalents (54) (167) (334)
Cash and cash equivalents, January 1 403 570 904
Cash and cash equivalents, December 31 349 403 570
SUPPLEMENTAL DISCLOSURE OF NONCASH FINANCING ACTIVITIES      
Financing of build-to-suit property 0 61 61
Common dividends issued in stock 53 55 42
Dividends declared but not paid 196 180 166
Parent Company [Member]      
Condensed Cash Flow Statements, Captions [Line Items]      
Net cash used in operating activities (178) (255) (260)
Dividends received from subsidiaries 175 350 300
Expenditures for property, plant and equipment (5) (43) (15)
Purchase of trust assets 0 (5) (4)
Decrease (increase) in loans to affiliates, net 457 (457) 627
Net cash used in investing activities 627 (155) 908
Common stock dividends paid (686) (628) (598)
Issuances of common stock 51 52 56
Repurchases of common stock (56) (74) (38)
Issuances of long-term debt 499 1,248 499
Payments on long-term debt (750) 0 (800)
Increase (decrease) in loans from affiliates, net 504 (230) 234
Tax benefit related to share-based compensation 0 52 0
Other (3) (9) (4)
Net cash provided by (used in) financing activities (441) 411 (651)
Increase (decrease) in cash and cash equivalents 8 1 (3)
Cash and cash equivalents, January 1 4 3 6
Cash and cash equivalents, December 31 12 4 3
SUPPLEMENTAL DISCLOSURE OF NONCASH FINANCING ACTIVITIES      
Financing of build-to-suit property 0 61 61
Common dividends issued in stock 53 55 42
Dividends declared but not paid $ 189 $ 174 $ 163