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Segment and Related Information - Income Statement Data (Details) - USD ($)
$ in Millions
3 Months Ended
Mar. 31, 2016
Mar. 31, 2015
Segment Reporting, Revenue Reconciling Item [Line Items]    
Revenues from external customers $ 1,173 $ 1,091
Operating income (loss) 311 178
Operating Segments | East    
Segment Reporting, Revenue Reconciling Item [Line Items]    
Revenues from external customers [1] 1,096 1,047
Operating income (loss) 392 231
Operating Segments | West    
Segment Reporting, Revenue Reconciling Item [Line Items]    
Revenues from external customers [1] 77 44
Operating income (loss) (28) (1)
Other    
Segment Reporting, Revenue Reconciling Item [Line Items]    
Operating income (loss) $ (53) $ (52)
[1] Includes unrealized gains and losses from derivatives. See Note 13 for additional information.