Inventory (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2016
Inventory Disclosure [Abstract]
Fuel oil	$ 312	$ 289
Coal/Lignite	471	334
Natural gas	12	28
Spare parts	437	413
Other	20	47
Total Inventory	1,252	$ 1,111
Inventory Write-down	$ 19