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Non-Wholly Owned Entities and Contingently Redeemable Noncontrolling Interest (Details) - USD ($)
$ in Millions
3 Months Ended 6 Months Ended
Jun. 30, 2016
Jun. 30, 2015
Jun. 30, 2016
Jun. 30, 2015
Feb. 12, 2016
Schedule of Equity Method Investments [Line Items]          
Expenditures for property, plant and equipment $ 313 $ 264 $ 548 $ 452  
Distribution to noncontrolling interest     19 18  
South Star [Member]          
Schedule of Equity Method Investments [Line Items]          
Expenditures for property, plant and equipment     $ 2 2  
Ownership interest in variable interest entity (as a percent)     85.00%    
South Star [Member] | Piedmont [Member]          
Schedule of Equity Method Investments [Line Items]          
Ownership interest to be sold (as a percent)         15.00%
Letter of agreement for sale of ownership interest         $ 160
Distribution to noncontrolling interest     $ 19 $ 18