Variable Interest Entities, or VIEs (Details) $ in Millions	Jun. 30, 2017 USD ($) facility MW	Dec. 31, 2016 USD ($)
Investments Accounted for by the Equity Method
Generation capacity (in MW) | MW	31,000
Equity investments in affiliates	$ 1,127	$ 1,120
Deficit Restoration Obligation	95
Current assets	84	87
Net property, plant and equipment	1,493	1,534
Other long-term assets	1,024	954
Total assets	2,601	2,575
Current liabilities	63	59
Long-term debt	424	442
Other long-term liabilities	187	183
Total liabilities	674	684
Noncontrolling interests	600	529
Net assets less noncontrolling interests	$ 1,327	$ 1,362
GenConn Energy LLC
Investments Accounted for by the Equity Method
Economic interest in equity method investments (as a percent)	50.00%
Power generation facilities | facility	2
Generation capacity (in MW) | MW	190
Equity investments in affiliates	$ 104