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Stock Plans - Additional Information (Detail) (USD $)
In Millions, except Share data, unless otherwise specified
3 Months Ended 9 Months Ended
Sep. 30, 2012
Sep. 30, 2011
Sep. 30, 2012
Sep. 30, 2011
Dec. 31, 2011
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Compensation costs $ 2.1   $ 2.1    
Stock options, outstanding 1,705,034   1,705,034   2,532,484
Compensation cost on stock-based compensation plans $ (0.2) $ 2.6 $ 5.1 $ 7.4  
REIT Conversion [Member]
         
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Stock option cancelled 167,500   167,500    
Weighted average exercise price $ 22.02   $ 22.02    
Equity incentive plan restricted stock [Member]
         
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Restricted stock award granted by Company     311,230    
Weighted-average grant-date fair value of restricted stock awards granted     $ 30.23    
Restricted stock award, outstanding 458,461   458,461   633,647
Restricted stock awards for certain management [Member]
         
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Restricted stock award granted by Company     104,500    
Weighted-average grant-date fair value of restricted stock awards granted     $ 39.88    
Possible vesting date     2015    
Risk-free rate of weighted-average grant date fair value     0.54%    
Expected life assumption for weighted average grant date fair value     3 years    
Restricted stock awards for certain management [Member] | Minimum [Member]
         
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Volatility of weighted-average grant date fair value     15.00%    
Restricted stock awards for certain management [Member] | Maximum [Member]
         
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Volatility of weighted-average grant date fair value     238.00%    
Restricted Stock Units (RSUs) [Member] | REIT Conversion [Member]
         
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Restricted stock units 401,000   401,000    
Weighted average grant date fair value $ 29.76   $ 29.76