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Goodwill and Other Intangibles - Components of Intangible Assets Subject to Amortization (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2019 | Dec. 31, 2018
Finite-lived Intangible Assets [Roll Forward] |  |
Balance at beginning of period | $ 1,742 | $ 2,080
Purchases | 42 | 61
Acquisition of businesses | 318 | 178
Usage | (21) | (27)
Write-off of fully amortized balances | (30) | (528)
Impairment | (2) | (6)
Other | 2 | (16)
Balance at end of period | 2,051 | 1,742
Less accumulated amortization | (1,262) | (1,151)
Net carrying amount | 789 | 591
Emission Allowances |  |
Finite-lived Intangible Assets [Roll Forward] |  |
Balance at beginning of period | 659 | 755
Purchases | 13 | 33
Acquisition of businesses | 0 | 0
Usage | (4) | (1)
Write-off of fully amortized balances | (8) | (107)
Impairment | 0 | (5)
Other | 2 | (16)
Balance at end of period | 662 | 659
Less accumulated amortization | (539) | (515)
Net carrying amount | 123 | 144
Fuel Contracts |  |
Finite-lived Intangible Assets [Roll Forward] |  |
Balance at beginning of period | 49 | 49
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 | 49 | 49
Less accumulated amortization | (45) | (45)
Net carrying amount | $ 4 | 4
Customer Contracts |  |
Finite-lived Intangible Assets [Roll Forward] |  |
Weighted average useful life, years | 2 years |
Customer Relationships |  |
Finite-lived Intangible Assets [Roll Forward] |  |
Balance at beginning of period | $ 478 | 768
Purchases | 0 | 0
Acquisition of businesses | 110 | 122
Usage | 0 | 0
Write-off of fully amortized balances | (13) | (411)
Impairment | (2) | (1)
Other | 0 | 0
Balance at end of period | 573 | 478
Less accumulated amortization | (345) | (314)
Net carrying amount | $ 228 | $ 164
Weighted average useful life, years | 7 years | 6 years
Marketing Partnerships |  |
Finite-lived Intangible Assets [Roll Forward] |  |
Balance at beginning of period | $ 131 | $ 88
Purchases | 0 | 0
Acquisition of businesses | 154 | 43
Usage | 0 | 0
Write-off of fully amortized balances | 0 | 0
Impairment | 0 | 0
Other | 0 | 0
Balance at end of period | 285 | 131
Less accumulated amortization | (75) | (61)
Net carrying amount | $ 210 | $ 70
Weighted average useful life, years | 9 years | 14 years
Trade Names |  |
Finite-lived Intangible Assets [Roll Forward] |  |
Balance at beginning of period | $ 345 | $ 342
Purchases | 0 | 0
Acquisition of businesses | 28 | 13
Usage | 0 | 0
Write-off of fully amortized balances | 0 | (10)
Impairment | 0 | 0
Other | 0 | 0
Balance at end of period | 373 | 345
Less accumulated amortization | (220) | (195)
Net carrying amount | $ 153 | $ 150
Weighted average useful life, years | 12 years | 7 years
Other |  |
Finite-lived Intangible Assets [Roll Forward] |  |
Balance at beginning of period | $ 80 | $ 78
Purchases | 29 | 28
Acquisition of businesses | 26 | 0
Usage | (17) | (26)
Write-off of fully amortized balances | (9) | 0
Impairment | 0 | 0
Other | 0 | 0
Balance at end of period | 109 | 80
Less accumulated amortization | (38) | (21)
Net carrying amount | $ 71 | $ 59

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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 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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- 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/disclosureRef -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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- Definition Amount before amortization of assets, excluding financial assets and goodwill, lacking physical substance with a finite life. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 928 -SubTopic 340 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6473545&loc=d3e61844-108004 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -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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- Definition The amount of impairment loss recognized in the period resulting from the write-down of the carrying amount of a finite-lived intangible asset to fair value. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=118932676&loc=d3e3602-108585 Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 30 -Section 50 -Paragraph 3 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=66006027&loc=d3e16373-109275 + Details
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