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Fair Value of Financial Instruments - Derivative Fair Value Measurements (Details) $ in Millions	Sep. 30, 2018 USD ($) $ / MWh	Dec. 31, 2017 USD ($) $ / MWh
Fair value, measurements, recurring
Liabilities
Derivative liabilities	$ 907	$ 680
Fair value, measurements, recurring | Level 3
Liabilities
Derivative liabilities	82	59
Commodity contracts
Assets
Derivative assets	1,022	744
Liabilities
Derivative liabilities	905	674
Commodity contracts | Fair value, measurements, recurring
Assets
Derivative assets	1,022	744
Liabilities
Derivative liabilities	905	674
Commodity contracts | Fair value, measurements, recurring | Level 3
Assets
Derivative assets	69	44
Liabilities
Derivative liabilities	82	59
Commodity contracts | Fair value, measurements, recurring | Level 3 | Power Contracts
Assets
Derivative assets	44	33
Liabilities
Derivative liabilities	$ 68	$ 47
Commodity contracts | Fair value, measurements, recurring | Level 3 | Power Contracts | Low
Fair Value Inputs / Range
Forward Market Price (per MWh) | $ / MWh	2	10
Commodity contracts | Fair value, measurements, recurring | Level 3 | Power Contracts | High
Fair Value Inputs / Range
Forward Market Price (per MWh) | $ / MWh	197	142
Commodity contracts | Fair value, measurements, recurring | Level 3 | Power Contracts | Weighted Average
Fair Value Inputs / Range
Forward Market Price (per MWh) | $ / MWh	23	24
Commodity contracts | Fair value, measurements, recurring | Level 3 | FTRs
Assets
Derivative assets	$ 25	$ 11
Liabilities
Derivative liabilities	$ 14	$ 12
Commodity contracts | Fair value, measurements, recurring | Level 3 | FTRs | Low
Fair Value Inputs / Range
Auction Prices (per MWh) | $ / MWh	(90)	(28)
Commodity contracts | Fair value, measurements, recurring | Level 3 | FTRs | High
Fair Value Inputs / Range
Auction Prices (per MWh) | $ / MWh	86	46
Commodity contracts | Fair value, measurements, recurring | Level 3 | FTRs | Weighted Average
Fair Value Inputs / Range
Auction Prices (per MWh) | $ / MWh	0	0

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