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Revenue Recognition, Narrative (Details) - USD ($) $ in Millions | 3 Months Ended
--- | ---
Mar. 31, 2018 | Mar. 31, 2017
Revenue recognition [Line Items] |  |
Cumulative effect of accounting change | $ 176 |
Bad debts expense | $ 19 | $ 24
Revenue recognition, practical expedient | false |
Capitalized contract cost |  |
Capitalized contract cost related to commission fees | $ 133 |
Amortization of contract cost assets | 27 |
Impairment of capitalized contract cost | $ 0 |
Contract cost practical expedient | true |
Retained Earnings [Member] |  |
Revenue recognition [Line Items] |  |
Cumulative effect of accounting change | $ 175 |
Revenue from contract with customer |  |
Revenue recognition [Line Items] |  |
Bad debts expense | 19 |
Revenue recognition | Retained Earnings [Member] |  |
Revenue recognition [Line Items] |  |
Cumulative effect of accounting change | $ 175 |

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- Definition Amount of accumulated amortization of asset recognized from cost incurred to obtain or fulfill contract with customer. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -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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- Definition Amount of accumulated impairment loss for asset recognized from cost incurred to obtain or fulfill contract with customer. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -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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- 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/presentationRef -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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- Definition Amount of expense related to write-down of receivables to the amount expected to be collected. Includes, but is not limited to, accounts receivable and notes receivable. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 606 -SubTopic 10 -Section 45 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=82913815&loc=SL49130534-203044 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 225 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03.5) -URI http://asc.fasb.org/extlink&oid=63488584&loc=d3e20235-122688 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3602-108585 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 606 -SubTopic 10 -Section 50 -Paragraph 4 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=82913847&loc=SL49130543-203045 + Details
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