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Fair Value Measurements (Details 1) - Level 3 - USD ($) $ in Millions | 9 Months Ended | 12 Months Ended
--- | --- | ---
Sep. 30, 2018 | Dec. 31, 2017
NUG contracts |  |
Reconciliation of changes in the fair value of FTRs contracts |  |
Beginning Balance, Derivative Assets | $ 0 | $ 1
Beginning Balance, Derivative Liabilities | (79) | (108)
Beginning Balance, Net | (79) | (107)
Unrealized gain (loss), Derivative Assets | 0 | 0
Unrealized loss, Derivative Liabilities | 2 | (10)
Unrealized loss, Net | 2 | (10)
Purchases, Derivative Assets | 0 | 0
Purchases, Derivative Liabilities | 0 | 0
Purchases, Net | 0 | 0
Settlements, Derivative Assets | 0 | (1)
Settlements, Derivative Liabilities | 25 | 39
Settlements, Net | 25 | 38
Ending Balance, Derivative Assets | 0 | 0
Ending Balance, Derivative Liabilities | (52) | (79)
Ending Balance, Net | (52) | (79)
FTRs |  |
Reconciliation of changes in the fair value of FTRs contracts |  |
Beginning Balance, Derivative Assets | 3 | 3
Beginning Balance, Derivative Liabilities | 0 | (1)
Beginning Balance, Net | 3 | 2
Unrealized gain (loss), Derivative Assets | 7 | 1
Unrealized loss, Derivative Liabilities | 2 | (1)
Unrealized loss, Net | 9 | 0
Purchases, Derivative Assets | 5 | 3
Purchases, Derivative Liabilities | (5) | 0
Purchases, Net | 0 | 3
Settlements, Derivative Assets | (5) | (4)
Settlements, Derivative Liabilities | 2 | 2
Settlements, Net | (3) | (2)
Ending Balance, Derivative Assets | 10 | 3
Ending Balance, Derivative Liabilities | (1) | 0
Ending Balance, Net | $ 9 | $ 3

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