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Stock-Based Compensation - Additional Information (Detail) (USD $)
 9 Months Ended 3 Months Ended 9 Months Ended 12 Months Ended 9 Months Ended 12 Months Ended
Sep. 30, 2012
Mar. 31, 2011
RSAs [Member]
May 18, 2011
2009 Equity Incentive Award Plan [Member]
Sep. 30, 2012
Performance-Based RSUs
Dec. 31, 2011
Performance-Based RSUs
Sep. 30, 2012
Performance-Based RSUs [Member]
Y
Dec. 31, 2011
Performance-Based RSUs [Member]
Dec. 31, 2011
Market-Based RSUs [Member]
Y
Dec. 31, 2009
Market-Based RSUs [Member]
Y
Dec. 31, 2011
Restricted Stock Units (RSUs) [Member]
Y
Dec. 31, 2009
Market-Based Stock Options [Member]
Y
Dec. 31, 2009
Performance-Based Stock Options [Member]
Y
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                        
Recognized compensation expense related to non-vested RSAs   $ 100,000                    
Grant date fair value of restricted stock vested 15,700,000                      
Aggregate intrinsic value of Restricted Stock Units expected to vest 34,800,000                      
Remaining weighted average vesting period, in years 1 year 3 months 18 days                      
Common stock reserved under the plan for future issuance     5,000,000                  
Increase in Common stock reserved under the plan for future issuance     7,000,000                  
RSUs granted during the period       282,850 323,600 191,594 195,156 90,000 98,000      
Estimated grant date fair value of restricted stock       4,900,000 5,000,000 3,000,000   1,100,000 900,000 3,000,000    
Service period, in years       1 year 1 year              
RSUs that remain outstanding       282,850   191,594   62,000 48,665 128,062    
Average derived service period, in years           3.00   3.93 4.35 3.00 4.88 3.71
Risk-free interest rate, RSUs               2.90% 3.50%      
Implied volatility               40.00% 50.00%   50.00%  
RSUs vested               28,000 29,335 64,909    
Approximate percentage of performance criteria achieved         90.00%              
Options granted, Shares outstanding                     294,076 32,674
Estimated grant date fair value of stock options                     $ 1,700,000 $ 100,000
Risk-free interest rate                     3.10%  
Stock options vested                     59,598 5,298
Stock options that remain outstanding                     131,118 15,540