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Fair Value of Financial Instruments - Derivative Fair Value Measurement (Details) $ in Millions | Dec. 31, 2017 USD ($) $ / MWh | Dec. 31, 2016 USD ($) $ / MWh
--- | --- | ---
Assets |  |
Derivative assets | $ 798 | $ 1,248
Liabilities |  |
Derivative liabilities | 752 | 1,376
Fair Value Inputs [Abstract] |  |
Cash collateral posted in support of energy risk management activities | 171 | 150
Cash collateral received in support of energy risk management activities | 37 | 81
Commodity contracts |  |
Assets |  |
Derivative assets | 745 | 1,199
Liabilities |  |
Derivative liabilities | 693 | 1,288
Fair Value Inputs [Abstract] |  |
Cash collateral posted in support of energy risk management activities | 73 | 13
Cash collateral received in support of energy risk management activities | 11 | 13
Commodity contracts | Fair Value, Measurements, Recurring |  |
Assets |  |
Derivative assets | 745 | 1,199
Liabilities |  |
Derivative liabilities | 693 | 1,288
Commodity contracts | Fair Value, Measurements, Recurring | Level 3 |  |
Assets |  |
Derivative assets | 45 | 90
Liabilities |  |
Derivative liabilities | 77 | 158
Commodity contracts | Fair Value, Measurements, Recurring | Power Contracts | Level 3 |  |
Assets |  |
Derivative assets | 34 | 39
Liabilities |  |
Derivative liabilities | $ 65 | $ 108
Commodity contracts | Fair Value, Measurements, Recurring | Power Contracts | Level 3 | Low |  |
Fair Value Inputs [Abstract] |  |
Forward Market Price (USD per MWh) | $ / MWh | 10 | 11
Commodity contracts | Fair Value, Measurements, Recurring | Power Contracts | Level 3 | High |  |
Fair Value Inputs [Abstract] |  |
Forward Market Price (USD per MWh) | $ / MWh | 142 | 104
Commodity contracts | Fair Value, Measurements, Recurring | Power Contracts | Level 3 | Weighted Average |  |
Fair Value Inputs [Abstract] |  |
Forward Market Price (USD per MWh) | $ / MWh | 33 | 31
Commodity contracts | Fair Value, Measurements, Recurring | Financial Transmission Rights | Level 3 |  |
Assets |  |
Derivative assets | $ 11 | $ 51
Liabilities |  |
Derivative liabilities | $ 12 | $ 50
Commodity contracts | Fair Value, Measurements, Recurring | Financial Transmission Rights | Level 3 | Low |  |
Fair Value Inputs [Abstract] |  |
Auction Prices (USD per MWh) | $ / MWh | (28) | (22)
Commodity contracts | Fair Value, Measurements, Recurring | Financial Transmission Rights | Level 3 | High |  |
Fair Value Inputs [Abstract] |  |
Auction Prices (USD per MWh) | $ / MWh | 46 | 17
Commodity contracts | Fair Value, Measurements, Recurring | Financial Transmission Rights | Level 3 | Weighted Average |  |
Fair Value Inputs [Abstract] |  |
Auction Prices (USD per MWh) | $ / MWh | 0 | 0

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- Definition Amount of obligation to return cash collateral under master netting arrangements that have not been offset against derivative assets. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 8 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=84234895&loc=d3e41678-113959 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 20 -Section 50 -Paragraph 3 -Subparagraph (d)(2) -URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 20 -Section 55 -Paragraph 13 -URI http://asc.fasb.org/extlink&oid=82849420&loc=SL20226016-175313 + Details
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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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