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Revenue - Narratives (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations      
Revenue form contract liability $ 741 $ 718 $ 297
Decrease in deferred revenue $ (67) $ (251) $ (78)
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2018-01-01      
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]      
Performance period 12 months    
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2018-04-01      
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]      
Revenue, Remaining Performance Obligation, Percentage 35.00%    
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2019-04-01      
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]      
Revenue, Remaining Performance Obligation, Percentage 25.00%    
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: (nil)      
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]      
Remaining performance obligation $ 19,500    
Minimum | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2019-01-01      
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]      
Performance period 13 months    
Maximum | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2019-01-01      
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]      
Performance period 24 months    
Disposal Group, Disposed of by Means Other than Sale, Not Discontinued Operations [Member]      
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations      
Decrease in contract assets with customer $ 2    
Decrease in deferred revenue $ 105