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Summary of Non-Vested Shares (Detail) (USD $)
3 Months Ended 6 Months Ended
Jun. 30, 2013
Mar. 31, 2013
Jun. 30, 2013
Performance Shares
Jun. 30, 2013
Performance Shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Number of Shares, Beginning balance 383,701 245,817    
Number of Shares, Granted 8,230 145,884 26,942 26,942
Number of Shares, Vested (8,230)      
Number of Shares, Forfeited   (8,000)    
Number of Shares, Ending Balance 383,701 383,701 26,942 26,942
Weighted Average Grant-Date Fair Value, Beginning balance $ 24.39 $ 20.23    
Weighted Average Grant-Date Fair Value, Granted $ 30.37 $ 31.27 $ 30.13  
Weighted Average Grant-Date Fair Value, Vested $ 30.37      
Weighted Average Grant-Date Fair Value, Forfeited   $ 22.17    
Weighted Average Grant-Date Fair Value, Ending balance $ 24.39 $ 24.39 $ 30.13 $ 30.13