Summary of Significant Accounting Policies - Reconciliation of Numerator and Denominator Used in Calculation of Basic and Diluted Net Income Per Share (Parenthetical) (Detail) - shares shares in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Earnings Per Share [Abstract]
Outstanding stock options and unvested restricted stock units excluded from computation of diluted EPS	0.5	0.7	1.3