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Revenue from Contracts with Customers - Contract Assets Rollforward (Details) - USD ($) $ in Millions | 3 Months Ended | 6 Months Ended
--- | --- | ---
Jun. 30, 2019 | Jun. 30, 2018 | Jun. 30, 2019 | Jun. 30, 2018
Capitalized Cost To Fulfill Customer Contracts |  |  |  |
Change in Capitalized Contract Costs |  |  |  |
Balance at beginning of period | $ 236 | $ 240 | $ 329 | $ 298
Additions | 336 | 341 | 682 | 711
Amortization | (357) | (353) | (796) | (785)
Impairment | 0 | 0 | 0 | 0
Foreign currency and other | 1 | (12) | 1 | (8)
Balance at end of period | 216 | 216 | 216 | 216
Capitalized Cost To Obtain Customer Contracts |  |  |  |
Change in Capitalized Contract Costs |  |  |  |
Balance at beginning of period | 155 | 144 | 156 | 145
Additions | 17 | 13 | 26 | 21
Amortization | (11) | (11) | (22) | (21)
Impairment | 0 | 0 | 0 | 0
Foreign currency and other | 0 | (2) | 1 | (1)
Balance at end of period | $ 161 | $ 144 | $ 161 | $ 144

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- Definition Amount of amortization expense for asset recognized from cost incurred to obtain or fulfill contract with customer. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 340 -SubTopic 40 -Section 50 -Paragraph 3 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=68051541&loc=SL49131252-203054 + Details
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- Definition Amount of impairment loss for asset recognized from cost incurred to obtain or fulfill contract with customer. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 340 -SubTopic 40 -Section 50 -Paragraph 3 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=68051541&loc=SL49131252-203054 + Details
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- Definition Amount, after accumulated amortization and accumulated impairment loss, of asset recognized from cost incurred to obtain or fulfill contract with customer. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 340 -SubTopic 40 -Section 50 -Paragraph 3 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=68051541&loc=SL49131252-203054 + Details
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