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Summary of Non-Vested Restricted Stock Activity (Detail) - Non-Vested Restricted Stock - $ / shares
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Shares, Non-vested beginning balance 2,898,890    
Shares, Granted 176,899    
Shares, Vested (80,550)    
Shares, Forfeited (10,000)    
Shares, Non-vested ending balance 2,985,239 2,898,890  
Weighted-Average Grant Date Fair Value, Non-vested beginning balance $ 54.13    
Weighted-Average Grant Date Fair Value, Granted 149.47 $ 130.01 $ 114.55
Weighted-Average Grant Date Fair Value, Vested 50.05    
Weighted-Average Grant Date Fair Value, Forfeited 134.71    
Weighted-Average Grant Date Fair Value, Non-vested ending balance $ 51.22 $ 54.13