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Goodwill and Other Intangibles - Components of Intangible Assets Subject to Amortization (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2017 | Dec. 31, 2016
Finite-lived Intangible Assets [Roll Forward] |  |
Balance at beginning of period | $ 3,661 | $ 3,721
Purchases | 63 | 47
Acquisition of businesses | 18 | 18
Usage | (38) | (45)
Write-off of fully amortized balances | (77) | (10)
Impairment | (26) | (64)
Other | (37) | (6)
Balance at end of period | 3,564 | 3,661
Less accumulated amortization | (1,818) | (1,688)
Net carrying amount | 1,746 | 1,973
Emission Allowances |  |
Finite-lived Intangible Assets [Roll Forward] |  |
Balance at beginning of period | 789 | 816
Purchases | 31 | 13
Acquisition of businesses | 0 | 0
Usage | (10) | (1)
Write-off of fully amortized balances | 0 | (10)
Impairment | (20) | (23)
Other | (23) | (6)
Balance at end of period | 767 | 789
Less accumulated amortization | (591) | (518)
Net carrying amount | 176 | 271
Energy Supply |  |
Finite-lived Intangible Assets [Roll Forward] |  |
Balance at beginning of period | 54 | 54
Purchases | 0 | 0
Acquisition of businesses | 0 | 0
Usage | 0 | 0
Write-off of fully amortized balances | (54) | 0
Impairment | 0 | 0
Other | 0 | 0
Balance at end of period | 0 | 54
Less accumulated amortization | 0 | (54)
Net carrying amount | 0 | 0
Fuel |  |
Finite-lived Intangible Assets [Roll Forward] |  |
Balance at beginning of period | 72 | 72
Purchases | 0 | 0
Acquisition of businesses | 0 | 0
Usage | 0 | 0
Write-off of fully amortized balances | (23) | 0
Impairment | 0 | 0
Other | 0 | 0
Balance at end of period | 49 | 72
Less accumulated amortization | (45) | (67)
Net carrying amount | 4 | 5
Customer |  |
Finite-lived Intangible Assets [Roll Forward] |  |
Balance at beginning of period | 16 | 16
Purchases | 0 | 0
Acquisition of businesses | 0 | 0
Usage | 0 | 0
Write-off of fully amortized balances | 0 | 0
Impairment | 0 | 0
Other | 0 | 0
Balance at end of period | 16 | 16
Less accumulated amortization | (9) | (8)
Net carrying amount | 7 | 8
Customer Relationships |  |
Finite-lived Intangible Assets [Roll Forward] |  |
Balance at beginning of period | 816 | 834
Purchases | 0 | 0
Acquisition of businesses | 18 | 0
Usage | 0 | 0
Write-off of fully amortized balances | 0 | 0
Impairment | 0 | (18)
Other | 0 | 0
Balance at end of period | 834 | 816
Less accumulated amortization | (698) | (663)
Net carrying amount | 136 | 153
Write-off of accumulated amortization included in impairment of finite lived assets |  | 10
Marketing Partnerships |  |
Finite-lived Intangible Assets [Roll Forward] |  |
Balance at beginning of period | 88 | 88
Purchases | 0 | 0
Acquisition of businesses | 0 | 0
Usage | 0 | 0
Write-off of fully amortized balances | 0 | 0
Impairment | 0 | 0
Other | 0 | 0
Balance at end of period | 88 | 88
Less accumulated amortization | (54) | (49)
Net carrying amount | 34 | 39
Trade Names |  |
Finite-lived Intangible Assets [Roll Forward] |  |
Balance at beginning of period | 342 | 342
Purchases | 0 | 0
Acquisition of businesses | 0 | 0
Usage | 0 | 0
Write-off of fully amortized balances | 0 | 0
Impairment | 0 | 0
Other | 0 | 0
Balance at end of period | 342 | 342
Less accumulated amortization | (182) | (159)
Net carrying amount | 160 | 183
PPA |  |
Finite-lived Intangible Assets [Roll Forward] |  |
Balance at beginning of period | 1,286 | 1,286
Purchases | 0 | 0
Acquisition of businesses | 0 |
Usage | 0 | 0
Write-off of fully amortized balances | 0 | 0
Impairment | (6) | 0
Other | 5 | 0
Balance at end of period | 1,285 | 1,286
Less accumulated amortization | (205) | (143)
Net carrying amount | 1,080 | 1,143
Other |  |
Finite-lived Intangible Assets [Roll Forward] |  |
Balance at beginning of period | 198 | 213
Purchases | 32 | 34
Acquisition of businesses | 0 | 18
Usage | (28) | (44)
Write-off of fully amortized balances | 0 | 0
Impairment | 0 | (23)
Other | (19) | 0
Balance at end of period | 183 | 198
Less accumulated amortization | (34) | (27)
Net carrying amount | $ 149 | 171
Write-off of accumulated amortization included in impairment of finite lived assets |  | $ 8

X
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- Definition Accumulated amount of amortization of assets, excluding financial assets and goodwill, lacking physical substance with a finite life. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 30 -Section 50 -Paragraph 2 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=66006027&loc=d3e16323-109275 + Details
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