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Stock Based Compensation - Information with Respect to Unvested Restricted Stock Awards Stock Option and Incentive Plan (Detail) - Restricted Stock [Member] - $ / shares
3 Months Ended 6 Months Ended
Jun. 30, 2015
Mar. 31, 2015
Jun. 30, 2014
Mar. 31, 2014
Jun. 30, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Beginning balance, shares 596,922 639,705 812,040 735,650 639,705
Granted, Number of Restricted Stock Awards 83,260     98,720  
Vested, Number of Restricted Stock Awards (16,000) (41,695) (32,000) (21,825)  
Forfeited, Number of Restricted Stock Awards (33,324) (1,088) (2,825) (505)  
Ending balance, shares 630,858 596,922 777,215 812,040 630,858
Nonvested, Weighted-Average Grant Date Fair Value, Beginning balance $ 27.75 $ 28.33 $ 28.37 $ 25.48 $ 28.33
Granted, Weighted-Average Grant Date Fair Value 44.46     48.42  
Vested, Weighted-Average Grant Date Fair Value 13.48 36.15 12.95 21.56  
Forfeited, Weighted-Average Grant Date Fair Value 23.20 48.42 43.43 32.20  
Nonvested, Weighted-Average Grant Date Fair Value, Ending balance $ 30.55 $ 27.75 $ 28.95 $ 28.37 $ 30.55