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Summary of Non-Vested Restricted Stock Activity (Detail) - Non-Vested Restricted Stock - $ / shares
12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Shares, Non-vested beginning balance 2,819,196    
Shares, Granted 183,144    
Shares, Vested (77,450)    
Shares, Forfeited (26,000)    
Shares, Non-vested ending balance 2,898,890 2,819,196  
Weighted-Average Grant Date Fair Value, Non-vested beginning balance $ 49.99    
Weighted-Average Grant Date Fair Value, Granted 130.01 $ 114.55 $ 96.84
Weighted-Average Grant Date Fair Value, Vested 63.26    
Weighted-Average Grant Date Fair Value, Forfeited 112.79    
Weighted-Average Grant Date Fair Value, Non-vested ending balance $ 54.13 $ 49.99