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Goodwill and Other Intangibles - Components of Intangible Assets Subject to Amortization (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2020 | Dec. 31, 2019
Finite-lived Intangible Assets [Roll Forward] |  |
Balance at beginning of period | $ 2,051 | $ 1,742
Purchases | 70 | 42
Acquisition of businesses | 22 | 318
Usage/Retirements | (35) | (21)
Write-off of fully amortized balances | (74) | (30)
Impairment | (14) | (2)
Other | 5 | 2
Balance at end of period | 2,025 | 2,051
Less accumulated amortization | (1,357) | (1,262)
Net carrying amount | 668 | 789
Renewables Energy Certificates |  |
Finite-lived Intangible Assets [Roll Forward] |  |
Carrying value | 28 | 18
Emission Allowances |  |
Finite-lived Intangible Assets [Roll Forward] |  |
Balance at beginning of period | 662 | 659
Purchases | 25 | 13
Acquisition of businesses | 0 | 0
Usage/Retirements | 0 | (4)
Write-off of fully amortized balances | (4) | (8)
Impairment | (14) | 0
Other | 3 | 2
Balance at end of period | 672 | 662
Less accumulated amortization | (563) | (539)
Net carrying amount | 109 | 123
Fuel Contracts |  |
Finite-lived Intangible Assets [Roll Forward] |  |
Balance at beginning of period | 49 | 49
Purchases | 0 | 0
Acquisition of businesses | 0 | 0
Usage/Retirements | 0 | 0
Write-off of fully amortized balances | 0 | 0
Impairment | 0 | 0
Other | 0 | 0
Balance at end of period | 49 | 49
Less accumulated amortization | (46) | (45)
Net carrying amount | 3 | 4
Customer Relationships |  |
Finite-lived Intangible Assets [Roll Forward] |  |
Balance at beginning of period | 573 | 478
Purchases | 0 | 0
Acquisition of businesses | 22 | 110
Usage/Retirements | 0 | 0
Write-off of fully amortized balances | (70) | (13)
Impairment | 0 | (2)
Other | 2 | 0
Balance at end of period | 527 | 573
Less accumulated amortization | (349) | (345)
Net carrying amount | $ 178 | $ 228
Weighted average useful life, years | 5 years | 7 years
Marketing Partnerships |  |
Finite-lived Intangible Assets [Roll Forward] |  |
Balance at beginning of period | $ 285 | $ 131
Purchases | 0 | 0
Acquisition of businesses | 0 | 154
Usage/Retirements | 0 | 0
Write-off of fully amortized balances | 0 | 0
Impairment | 0 | 0
Other | 0 | 0
Balance at end of period | 285 | 285
Less accumulated amortization | (99) | (75)
Net carrying amount | 186 | $ 210
Weighted average useful life, years |  | 9 years
Trade Names |  |
Finite-lived Intangible Assets [Roll Forward] |  |
Balance at beginning of period | 373 | $ 345
Purchases | 0 | 0
Acquisition of businesses | 0 | 28
Usage/Retirements | 0 | 0
Write-off of fully amortized balances | 0 | 0
Impairment | 0 | 0
Other | 0 | 0
Balance at end of period | 373 | 373
Less accumulated amortization | (247) | (220)
Net carrying amount | 126 | $ 153
Weighted average useful life, years |  | 12 years
Other(b) |  |
Finite-lived Intangible Assets [Roll Forward] |  |
Balance at beginning of period | 109 | $ 80
Purchases | 45 | 29
Acquisition of businesses | 0 | 26
Usage/Retirements | (35) | (17)
Write-off of fully amortized balances | 0 | (9)
Impairment | 0 | 0
Other | 0 | 0
Balance at end of period | 119 | 109
Less accumulated amortization | (53) | (38)
Net carrying amount | $ 66 | $ 71
Energy Supply Contract |  |
Finite-lived Intangible Assets [Roll Forward] |  |
Weighted average useful life, years |  | 2 years

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- Definition This element represents other additions or reductions to finite-lived intangible assets during the period. + References No definition available. + Details
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- Definition This element represents the finite-lived intangible assets purchased during the period. + References No definition available. + Details
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- Definition The usage and retirement of finite-lived intangible assets during the period. + References No definition available. + Details
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- Definition Weighted average amortization period of finite-lived intangible assets acquired either individually or as part of a group of assets, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 30 -Section 50 -Paragraph 1 -Subparagraph (a)(3) -URI http://asc.fasb.org/extlink&oid=66006027&loc=d3e16265-109275 + Details
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