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Stock-based Compensation (Details) (USD $)
In Thousands, except Per Share data, unless otherwise specified
3 Months Ended 12 Months Ended
Mar. 31, 2012
Mar. 31, 2011
Dec. 31, 2011
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Total stock-based compensation expense $ 4,362   $ 2,729
Additional disclosure [Abstract]      
Unrecognized compensation expense 12,117    
Weighted Average Remaining Vesting Period 2.85    
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]      
Unrecognized compensation expense 12,117    
Total unrecognized stock-based compensation expense 12,117    
Performance-based Restricted Stock Units [Member]
     
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Total stock-based compensation expense 2,701   877
Additional disclosure [Abstract]      
Unrecognized compensation expense 9,152    
Weighted Average Remaining Vesting Period 2.86    
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]      
Unrecognized compensation expense 9,152    
Total unrecognized stock-based compensation expense 9,152    
Stock Options [Member]
     
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Total stock-based compensation expense 1,128   1,358
Additional disclosure [Abstract]      
Unrecognized compensation expense 2,965    
Weighted Average Remaining Vesting Period 2.84    
Share-based payment award options outstanding [Roll Forward]      
Beginning Balance (in shares) 3,408    
Granted (in shares) 88    
Forfeited (in shares) 0    
Exercised (in shares) (494)    
Ending balance (in shares) 3,002   3,408
Options exercisable (in shares) 2,019    
Unvested stock options additional disclosure [Abstract]      
Beginning of period weighted average grant date fair value (dollars per share) $ 11.15    
Granted weighted average grant date fair value (dollars per share) $ 71.17    
Forfeited weighted average grant date fair value (dollars per share) $ 0.00    
Exercised weighted average grant date fair value (dollars per share) $ 14.82    
Ending of period weighted average grant date fair value (dollars per share) $ 15.06   $ 11.15
Options exercisable Weighted average grant date fair value $ 13.75    
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]      
Beginning balance weighted average exercise price (dollars per share) $ 13.47    
Granted weighted average exercise price (dollars per share) $ 28.65    
Exercised weighted average exercise price (dollars per share) $ 15.59    
Terminated weighted average exercise price (dollars per share) $ 13.52    
Ending balance weighted average exercise price (dollars per share) $ 13.31   $ 13.47
Options exercisable weighted average exercise price (dollars per share) $ 13.75    
Ending balance weighted average remaining contractual term (in years) 6.43    
Options exercisable weighted average remaining contractual term (in years) 6.08    
Ending balance aggregate intrinsic value 208,134    
Options exercisable aggregate intrinsic value 142,704    
Unrecognized compensation expense 2,965    
Total intrinsic value of options exercised 34,387 35,926  
Total unrecognized stock-based compensation expense 2,965    
Restricted Stock Units And Deferred Stock Units [Member]
     
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Total stock-based compensation expense 533   494
Amended And Restated 2003 Equity Incentive Plan [Member] | Performance-based Restricted Stock Units [Member]
     
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Maximum number of common stock shares to be issued (in shares) 403 443  
Share-based payment award [Roll forward]      
Beginning balance (in shares) 285    
Granted (in shares) 134    
Vested (in shares) 0    
Forfeited (in shares) 0    
Ending balance (in shares) 419    
Additional disclosure [Abstract]      
Weighted average grant date fair value, beginning balance (in dollars per share) $ 37.93    
Weighted average grant date fair value, granted (in dollars per share) $ 71.52    
Weighted average grant date fair value, vested (in dollars per share) $ 0.00    
Weighted average grant date fair value, forfeited (in dollars per share) $ 0.00    
Weighted average grant date fair value, ending balance (in dollars per share) $ 48.67    
Aggregate intrinsic value of units outstanding $ 35,359