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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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- Definition Fair value, before effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets elected not to be offset. Excludes assets not subject to a master netting arrangement. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 20 -Section 50 -Paragraph 3 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4B -Subparagraph (a),(c) -URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5624163-113959 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 20 -Section 55 -Paragraph 7 -URI http://asc.fasb.org/extlink&oid=82849420&loc=SL20226000-175313 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 860 -SubTopic 20 -Section 50 -Paragraph 4D -Subparagraph (c)(2) -URI http://asc.fasb.org/extlink&oid=82924980&loc=SL51823488-111719 + Details
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