Derivative Instruments (Details) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 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	845	405
Environmental Credits [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Mass | t	13	12
Interest Rate Hedging Instruments
Derivative [Line Items]
Derivative, Notional Amount | $	$ 4,600	$ 4,600
Short [Member] | Power [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Energy Measure | MWh	147	119