Derivative Instruments - Notional Amount of Derivative Instruments (Details) $ in Billions		6 Months Ended	12 Months Ended
		Jun. 30, 2020 USD ($) MMBTU MWh t	Dec. 31, 2019 USD ($) MMBTU MWh t
Derivative [Line Items]
Derivative, amount of hedged item	[1]	$ 2.4
Power
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Energy Measure | MWh		242	184
Natural Gas Contracts
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Energy Measure | MMBTU		1,165	1,063
Environmental credits
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Mass | t		36	26
Interest rate hedging instruments
Derivative [Line Items]
Derivative, notional amount	[1]	$ 7.0	$ 4.8

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