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Stockholders' Equity - Summary of Number of PSUs Granted Based on Adjusted EBITDA and Grant Date Fair Value (Detail) - PSUs – Adjusted EBITDA - $ / shares
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Number of units granted (in shares) 99,522 165,391 158,964
Fair value on grant date (in usd per share) $ 45.21 $ 43.09 $ 46.05