Derivative Instruments (Details) $ in Millions	12 Months Ended
	Dec. 31, 2018 USD ($) MMBTU MWh t	Dec. 31, 2017 USD ($) MMBTU MWh t
Natural Gas [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Energy Measure | MMBTU	(1,045)	(405)
Environmental Credits [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Mass | t	13	12
Interest Rate Contract [Member]
Derivative [Line Items]
Derivative, Notional Amount | $	$ 4,500	$ 4,600
Short [Member] | Power [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Energy Measure | MWh	(161)	(119)