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Summary of Non-Vested Restricted Stock Unit Activity (Detail) (USD $)
12 Months Ended
Dec. 31, 2012
Schedule of Share based Compensation Arrangements by Share based Payment Award, Equity Instruments, Other Than Options, Restricted Stock Units [Line Items]  
Non-vested units as of December 31, 2012 875,000
Unvested restricted stock units
 
Schedule of Share based Compensation Arrangements by Share based Payment Award, Equity Instruments, Other Than Options, Restricted Stock Units [Line Items]  
Non-vested units as of December 31, 2011 570,904
Shares, Granted 612,270
Shares, Vested (205,980)
Shares, Forfeited (78,245)
Non-vested units as of December 31, 2012 898,949
Weighted Average Grant Date Fair Value, Non-vested units as of December 31, 2011 $ 24.54
Weighted Average Grant Date Fair Value, granted $ 37.81
Weighted Average Grant Date Fair Value, vested $ 24.85
Weighted Average Grant Date Fair Value, forfeited $ 28.11
Weighted Average Grant Date Fair Value, Non-vested units as of December 31, 2012 $ 33.10