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Business Combinations - Schedule of Preliminary Allocations of Purchase to Certain Tangible and Intangible Assets Acquired and Goodwill (Detail) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Business Acquisition [Line Items]      
Total cost of acquisitions, net of cash acquired $ 854 $ (1,887) $ (32)
Goodwill 11,850 10,727 $ 720
Several Individually Immaterial Acquisitions [Member]      
Business Acquisition [Line Items]      
Total cost of acquisitions, net of cash acquired 923 136  
Goodwill 581 88  
Portion of goodwill deductible for income tax purposes 235    
Total intangible assets 392 48  
Several Individually Immaterial Acquisitions [Member] | Client Relationships [Member]      
Business Acquisition [Line Items]      
Total intangible assets $ 285 31  
Several Individually Immaterial Acquisitions [Member] | Client Relationships [Member] | Minimum [Member]      
Business Acquisition [Line Items]      
Amortization Period 6 years    
Several Individually Immaterial Acquisitions [Member] | Client Relationships [Member] | Maximum [Member]      
Business Acquisition [Line Items]      
Amortization Period 16 years    
Several Individually Immaterial Acquisitions [Member] | Backlog [Member]      
Business Acquisition [Line Items]      
Total intangible assets $ 15 7  
Several Individually Immaterial Acquisitions [Member] | Backlog [Member] | Minimum [Member]      
Business Acquisition [Line Items]      
Amortization Period 1 year    
Several Individually Immaterial Acquisitions [Member] | Backlog [Member] | Maximum [Member]      
Business Acquisition [Line Items]      
Amortization Period 4 years    
Several Individually Immaterial Acquisitions [Member] | Non-compete Agreements [Member]      
Business Acquisition [Line Items]      
Total intangible assets $ 14 9  
Several Individually Immaterial Acquisitions [Member] | Non-compete Agreements [Member] | Minimum [Member]      
Business Acquisition [Line Items]      
Amortization Period 2 years    
Several Individually Immaterial Acquisitions [Member] | Non-compete Agreements [Member] | Maximum [Member]      
Business Acquisition [Line Items]      
Amortization Period 5 years    
Several Individually Immaterial Acquisitions [Member] | Software and related assets [Member]      
Business Acquisition [Line Items]      
Total intangible assets $ 61 $ 1  
Several Individually Immaterial Acquisitions [Member] | Software and related assets [Member] | Minimum [Member]      
Business Acquisition [Line Items]      
Amortization Period 2 years    
Several Individually Immaterial Acquisitions [Member] | Software and related assets [Member] | Maximum [Member]      
Business Acquisition [Line Items]      
Amortization Period 9 years    
Several Individually Immaterial Acquisitions [Member] | Finite Lived Trade Name [Member]      
Business Acquisition [Line Items]      
Total intangible assets $ 17    
Several Individually Immaterial Acquisitions [Member] | Finite Lived Trade Name [Member] | Minimum [Member]      
Business Acquisition [Line Items]      
Amortization Period 1 year    
Several Individually Immaterial Acquisitions [Member] | Finite Lived Trade Name [Member] | Maximum [Member]      
Business Acquisition [Line Items]      
Amortization Period 17 years