Annual report pursuant to Section 13 and 15(d)

Summary of Potential Number of Units that Could Vest Under Restricted Stock Unit Awards (Parenthetical) (Detail)

v3.8.0.1
Summary of Potential Number of Units that Could Vest Under Restricted Stock Unit Awards (Parenthetical) (Detail) - $ / shares
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Restricted Stock Units (RSUs)      
Schedule Of Restricted Stock Unit [Line Items]      
Weighted Average Grant Date Fair Value, Granted $ 42.37 $ 29.83 $ 43.19