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Share-Based Compensation - Summary of RSUs Outstanding Under LTIP (Detail)
3 Months Ended
Mar. 31, 2018
$ / shares
shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Nonvested as of January 1, 2018 218,786
Granted 71,277
Vested (19,277)
Forfeited (30,793)
Nonvested as of March 31, 2018 239,993
Nonvested as of January 1, 2018, Weighted-Average Grant-Date Fair Value | $ / shares $ 52.68
Weighted-Average Grant-Date Fair Value, Granted | $ / shares 73.86
Weighted-Average Grant-Date Fair Value, Vested | $ / shares 45.85
Weighted-Average Grant-Date Fair Value, Forfeited | $ / shares 45.85
Nonvested as of March 31, 2018, Weighted-Average Grant-Date Fair Value | $ / shares $ 53.89
Performance and Service Required [Member]  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Nonvested as of January 1, 2018 200,572
Granted 63,122
Vested (19,277)
Forfeited (30,793)
Nonvested as of March 31, 2018 213,624
Service Required [Member]  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Nonvested as of January 1, 2018 18,214
Granted 8,155
Vested 0
Forfeited 0
Nonvested as of March 31, 2018 26,369