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Restructuring - Additional Information (Detail) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2016
Sep. 30, 2017
Sep. 30, 2016
Restructuring Cost and Reserve [Line Items]      
Merger related costs $ 4   $ 13
2017 Plan [Member]      
Restructuring Cost and Reserve [Line Items]      
Severance and related costs   $ 35  
Merger related costs   6  
2017 Plan [Member] | Cost Reductions Due to Changing Market Conditions [Member]      
Restructuring Cost and Reserve [Line Items]      
Severance and related costs   $ 29