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Nature of Operations and Basis of Presentation - Narrative (Details)
$ in Millions
3 Months Ended
Mar. 31, 2018
USD ($)
Segment
Mar. 31, 2017
USD ($)
Jan. 01, 2018
USD ($)
Dec. 31, 2017
USD ($)
Organization, Consolidation and Presentation of Financial Statements [Abstract]        
Number of reportable segments | Segment 4      
Disaggregation of Revenue [Line Items]        
Retained income $ 10,427     $ 10,025
Revenue from contract with customer, excluding assessed tax 1,567 $ 1,453    
Contract asset 28     $ 17
Accounting Standards Update 2014-09 | Difference between Revenue Guidance in Effect before and after Topic 606        
Disaggregation of Revenue [Line Items]        
Retained income     $ 35  
Revenue from contract with customer, excluding assessed tax 3      
Intersegment revenue 28 $ 25    
Accounts receivable, net     $ 1,319  
Unearned revenue 710      
Revenue expected to be recognized as of March 31, 2018 1,200      
Capitalized contract costs $ 81      
Minimum | Accounting Standards Update 2014-09 | Difference between Revenue Guidance in Effect before and after Topic 606        
Disaggregation of Revenue [Line Items]        
Expected timing of satisfaction of remaining performance obligation 12 months      
Maximum | Accounting Standards Update 2014-09 | Difference between Revenue Guidance in Effect before and after Topic 606        
Disaggregation of Revenue [Line Items]        
Expected timing of satisfaction of remaining performance obligation 24 months