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Fair Value of Financial Instruments - Derivative Fair Value Measurements (Details) $ in Millions	Mar. 31, 2021 USD ($) $ / MWh	Dec. 31, 2020 USD ($) $ / MWh
Liabilities
Commodity contracts	$ 2,440	$ 884
Level 3
Liabilities
Commodity contracts	105	155
Commodity contracts
Assets
Derivative assets	2,824	821
Liabilities
Commodity contracts	2,438	884
Commodity contracts | Level 3
Assets
Derivative assets	264	139
Liabilities
Commodity contracts	105	155
Commodity contracts | Level 3 | Natural Gas Contracts
Assets
Derivative assets	3
Liabilities
Commodity contracts	$ 0
Commodity contracts | Level 3 | Natural Gas Contracts | Low
Fair Value Inputs / Range
Forward Market Price (per MWh) | $ / MWh	1
Commodity contracts | Level 3 | Natural Gas Contracts | High
Fair Value Inputs / Range
Forward Market Price (per MWh) | $ / MWh	16
Commodity contracts | Level 3 | Natural Gas Contracts | Weighted Average
Fair Value Inputs / Range
Forward Market Price (per MWh) | $ / MWh	14
Commodity contracts | Level 3 | Power Contracts
Assets
Derivative assets	$ 234	111
Liabilities
Commodity contracts	$ 91	$ 143
Commodity contracts | Level 3 | Power Contracts | Low
Fair Value Inputs / Range
Forward Market Price (per MWh) | $ / MWh	1	10
Commodity contracts | Level 3 | Power Contracts | High
Fair Value Inputs / Range
Forward Market Price (per MWh) | $ / MWh	237	105
Commodity contracts | Level 3 | Power Contracts | Weighted Average
Fair Value Inputs / Range
Forward Market Price (per MWh) | $ / MWh	29	21
Commodity contracts | Level 3 | FTRs
Assets
Derivative assets	$ 27	$ 28
Liabilities
Commodity contracts	$ 14	$ 12
Commodity contracts | Level 3 | FTRs | Low
Fair Value Inputs / Range
Auction Prices (per MWh) | $ / MWh	(33)	(28)
Commodity contracts | Level 3 | FTRs | High
Fair Value Inputs / Range
Auction Prices (per MWh) | $ / MWh	320	43
Commodity contracts | Level 3 | FTRs | Weighted Average
Fair Value Inputs / Range
Auction Prices (per MWh) | $ / MWh	0	0

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