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Summary of Restricted Stock Activity (Detail) (USD $)
3 Months Ended
Mar. 31, 2013
Shares  
Unvested at December 31, 2012 149,667
Granted 141,012
Vested (52,679)
Unvested at March 31, 2013 238,000
Weighted-Average Grant Date Fair Value  
Unvested at December 31, 2012 $ 8.21
Granted $ 26.03
Vested $ 10.36
Unvested at March 31, 2013 $ 18.29