Annual report pursuant to Section 13 and 15(d)

Summary of Significant Accounting Policies - Schedule of Weighted Average Fair Value of Options Granted (Detail)

v3.8.0.1
Summary of Significant Accounting Policies - Schedule of Weighted Average Fair Value of Options Granted (Detail) - $ / shares
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Schedule Of Weighted Average Assumptions For Fair Values Of Stock Options [Line Items]      
Fair value     $ 9.90
Performance Based Nonvested [Member]      
Schedule Of Weighted Average Assumptions For Fair Values Of Stock Options [Line Items]      
Expected life (years) 3 years 3 years 3 years
Risk free interest rate 1.79% 1.53% 1.33%
Expected volatility 19.92% 19.37% 18.88%
Fair value $ 82.06 $ 80.24 $ 101.43