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Fair Value Measurements (Details 1) - Level 3 - USD ($) $ in Millions | 6 Months Ended | 12 Months Ended
--- | --- | ---
Jun. 30, 2017 | Dec. 31, 2016
NUG contracts |  |
Reconciliation of changes in the fair value of FTRs contracts |  |
Beginning Balance, Derivative Assets | $ 1 | $ 1
Beginning Balance, Derivative Liabilities | (108) | (137)
Beginning Balance, Net | (107) | (136)
Unrealized loss, Derivative Assets | 0 | 2
Unrealized loss, Derivative Liabilities | (10) | (17)
Unrealized loss, Net | (10) | (15)
Purchases, Derivative Assets | 0 | 0
Purchases, Derivative Liabilities | 0 | 0
Purchases, Net | 0 | 0
Settlements, Derivative Assets | (1) | (2)
Settlements, Derivative Liabilities | 19 | 46
Settlements, Net | 18 | 44
Ending Balance, Derivative Assets | 0 | 1
Ending Balance, Derivative Liabilities | (99) | (108)
Ending Balance, Net | (99) | (107)
FTRs |  |
Reconciliation of changes in the fair value of FTRs contracts |  |
Beginning Balance, Derivative Assets | 7 | 8
Beginning Balance, Derivative Liabilities | (6) | (13)
Beginning Balance, Net | 1 | (5)
Unrealized loss, Derivative Assets | 0 | (6)
Unrealized loss, Derivative Liabilities | (2) | (4)
Unrealized loss, Net | (2) | (10)
Purchases, Derivative Assets | 4 | 16
Purchases, Derivative Liabilities | (1) | (7)
Purchases, Net | 3 | 9
Settlements, Derivative Assets | (7) | (11)
Settlements, Derivative Liabilities | 7 | 18
Settlements, Net | 0 | 7
Ending Balance, Derivative Assets | 4 | 7
Ending Balance, Derivative Liabilities | (2) | (6)
Ending Balance, Net | 2 | 1
FTRs | FES |  |
Reconciliation of changes in the fair value of FTRs contracts |  |
Beginning Balance, Derivative Assets | 4 | 5
Beginning Balance, Derivative Liabilities | (5) | (11)
Beginning Balance, Net | (1) | (6)
Unrealized loss, Derivative Assets | 0 | (4)
Unrealized loss, Derivative Liabilities | (1) | (3)
Unrealized loss, Net | (1) | (7)
Purchases, Derivative Assets | 1 | 10
Purchases, Derivative Liabilities | (1) | (5)
Purchases, Net | 0 | 5
Settlements, Derivative Assets | (4) | (7)
Settlements, Derivative Liabilities | 5 | 14
Settlements, Net | 1 | 7
Ending Balance, Derivative Assets | 1 | 4
Ending Balance, Derivative Liabilities | (2) | (5)
Ending Balance, Net | $ (1) | $ (1)

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