Severance Costs - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Restructuring And Related Activities [Abstract]
Severance expense	$ 46
Reduction to expense for stock-based compensation forfeitures	9
Severance cost paid	21
Accrued severance cost	$ 31	$ 6
Expected period for payment for severance cost	We expect to pay in 2018.