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Stock-based Compensation (Details) (Restricted Stock Units [Member], USD $)
9 Months Ended
Sep. 30, 2012
Restricted Stock Units [Member]
 
Summary of restricted stock units activity  
Outstanding at December 31, 2011, Number of Shares 576,704
Outstanding at December 31, 2011, Weighted Average Grant Date Fair Value $ 24.16
Granted, Number of Shares 80,133
Granted, Weighted Average Grant Date Fair Value $ 81.32
Forfeited, Number of Shares (3,133)
Forfeited, Weighted Average Grant Date Fair Value $ 42.91
Settled, Number of Shares (374,675)
Settled, Weighted Average Grant Date Fair Value $ 13.35
Outstanding at September 30, 2012, Number of Shares 279,029
Outstanding at September 30, 2012, Weighted Average Grant Date Fair Value $ 54.94