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Stock-Based Compensation - Summary of Performance Share Units (Detail) (Performance Share Unit Awards [Member], USD $)
3 Months Ended
Mar. 31, 2013
Performance Share Unit Awards [Member]
 
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Non-vested shares outstanding, Beginning Balance 155,220
Granted 135,620
Vested   
Forfeited   
Non-vested shares outstanding, Ending Balance 290,840
Non-vested PSUs outstanding, Beginning Balance, Weighted Average Grant Date Fair Value per Unit $ 26.22
Granted, Weighted Average Grant Date Fair Value per Unit $ 42.01
Non-vested PSUs outstanding, Ending Balance, Weighted Average Grant Date Fair Value per Unit $ 33.58