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Share-Based Compensation - Weighted-Average Assumptions Used for Grants of Stock Options and SARs (Detail) - SARs - USD ($)
$ / shares in Units, $ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Expected dividend yield 0.00% 0.00% 0.00%
Forfeiture rate 9.37% 9.01% 10.24%
Volatility 35.49% 43.14% 33.98%
Risk free interest rate 1.77% 1.29% 1.53%
Range of interest rates, minimum 0.71% 0.25% 0.02%
Range of interest rates, maximum 2.41% 1.75% 2.14%
Expected weighted-average life 4 years 1 month 17 days 5 years 3 months 29 days 5 years 3 months 25 days
Fair value of SARs granted (in millions) $ 12,010 $ 12,650 $ 11,630
Weighted-average grant date fair value of SARs granted (per underlying share) (in USD per share) $ 29.86 $ 20.18 $ 35.00