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Summary of Non-Vested Restricted Stock Activity (Detail) - Non-Vested Restricted Stock - $ / shares
12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Shares, Non-vested beginning balance 2,643,717    
Shares, Granted 200,479    
Shares, Vested (20,000)   0
Shares, Forfeited (5,000)    
Shares, Non-vested ending balance 2,819,196 2,643,717  
Weighted-Average Grant Date Fair Value, Non-vested beginning balance $ 45.21    
Weighted-Average Grant Date Fair Value, Granted 114.55 $ 96.84 $ 80.21
Weighted-Average Grant Date Fair Value, Vested 49.36    
Weighted-Average Grant Date Fair Value, Forfeited 110.36    
Weighted-Average Grant Date Fair Value, Non-vested ending balance $ 49.99 $ 45.21