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Share-based Compensation - Summary of Stock-Settled SARs Activity (Detail) - Stock-Settled SARs Activity [Member]
12 Months Ended
Dec. 31, 2017
$ / shares
shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Shares, Outstanding at January 1, 2017 | shares 1,982,695
Exercised, Shares | shares (486,454)
Expired or forfeited, Shares | shares (186,089)
Shares, Outstanding at December 31, 2017 | shares 1,310,152
Vested or expected to vest as of year end, Shares | shares 1,236,514
Exercisable at end of year, Shares | shares 891,067
Weighted-Average Exercise Price, Outstanding at January 1, 2017 | $ / shares $ 34.80
Exercised, Weighted-Average Exercise Price | $ / shares 30.77
Expired or forfeited, Weighted-Average Exercise Price | $ / shares 40.75
Weighted-Average Exercise Price, Outstanding at December 31, 2017 | $ / shares 35.45
Vested or expected to vest as of year end, Weighted-Average Exercise Price | $ / shares 19.80
Exercisable at end of year, Weighted-Average Exercise Price | $ / shares $ 22.80