Discontinued Operations and Dispositions - Rockford Disposition (Details) $ in Millions			3 Months Ended		6 Months Ended
	Jul. 12, 2016 USD ($)	May 13, 2016 USD ($)	Jun. 30, 2017 USD ($) MW	Jun. 30, 2016 USD ($)	Jun. 30, 2017 USD ($) MW	Jun. 30, 2016 USD ($)	Dec. 31, 2016 USD ($)	May 12, 2016 USD ($) MW
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Generation capacity (in MW) | MW			31,000		31,000
Impairment losses			$ 63	$ 56	$ 63	$ 56
Current assets - discontinued operations			0		0		$ 1,919
Non-current assets - discontinued operations			$ 0		$ 0		$ 2,961
Rockford
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Percentage of ownership								100.00%
Impairment losses				17
Proceeds from sale of property, plant, and equipment		$ 55
Rockford | Disposed of by Sale
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Percentage of ownership								100.00%
Cash consideration								$ 55
Generation capacity (in MW) | MW								450
Impairment losses				17
Current assets - discontinued operations				2		2
Non-current assets - discontinued operations				$ 54		$ 54
Proceeds from sale of property, plant, and equipment	$ 56
Adjustments for base residual auction results	$ 1