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Fair Value of Financial Instruments - Derivative Fair Value Measurements (Details) $ in Millions	Jun. 30, 2018 USD ($) $ / MWh	Dec. 31, 2017 USD ($) $ / MWh
Assets
Derivative assets	$ 1,277	$ 798
Liabilities
Derivative liabilities	994	752
Fair value, measurements, recurring
Liabilities
Derivative liabilities	994	752
Fair value, measurements, recurring | Level 3
Liabilities
Derivative liabilities	347	77
Commodity contracts
Assets
Derivative assets	1,169	745
Liabilities
Derivative liabilities	971	693
Commodity contracts | Fair value, measurements, recurring
Assets
Derivative assets	1,169	745
Liabilities
Derivative liabilities	971	693
Commodity contracts | Fair value, measurements, recurring | Level 3
Assets
Derivative assets	500	45
Liabilities
Derivative liabilities	347	77
Commodity contracts | Fair value, measurements, recurring | Level 3 | Power Contracts
Assets
Derivative assets	481	34
Liabilities
Derivative liabilities	$ 330	$ 65
Commodity contracts | Fair value, measurements, recurring | Level 3 | Power Contracts | Low
Fair Value Inputs / Range
Forward Market Price (per MWh) | $ / MWh	6	10
Commodity contracts | Fair value, measurements, recurring | Level 3 | Power Contracts | High
Fair Value Inputs / Range
Forward Market Price (per MWh) | $ / MWh	198	142
Commodity contracts | Fair value, measurements, recurring | Level 3 | Power Contracts | Weighted Average
Fair Value Inputs / Range
Forward Market Price (per MWh) | $ / MWh	35	33
Commodity contracts | Fair value, measurements, recurring | Level 3 | FTRs
Assets
Derivative assets	$ 19	$ 11
Liabilities
Derivative liabilities	$ 17	$ 12
Commodity contracts | Fair value, measurements, recurring | Level 3 | FTRs | Low
Fair Value Inputs / Range
Auction Prices (per MWh) | $ / MWh	(48)	(28)
Commodity contracts | Fair value, measurements, recurring | Level 3 | FTRs | High
Fair Value Inputs / Range
Auction Prices (per MWh) | $ / MWh	47	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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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 50
-Paragraph 3
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