Annual report pursuant to Section 13 and 15(d)

Share-Based Compensation and Employee Benefit Plans (Details)

v2.4.0.6
Share-Based Compensation and Employee Benefit Plans (Details) (USD $)
In Millions, except Share data, unless otherwise specified
 12 Months Ended 1 Months Ended 12 Months Ended
Dec. 31, 2011
Amended and Restated 2008 Omnibus Equity and Incentive Plan
Maximum
Dec. 31, 2011
Options
Dec. 31, 2011
Restricted shares
Dec. 31, 2010
Restricted shares
Dec. 31, 2009
Restricted shares
Mar. 31, 2011
Performance share units
Mar. 31, 2010
Performance share units
Dec. 31, 2011
Performance share units
Rank
Dec. 31, 2010
Performance share units
Share-Based Compensation                  
Number of common shares of beneficial interest authorized to be issued 5,900,000                
Expiration period (in years)   P10Y              
Shares                  
Unvested at the beginning of the period (in shares)     669,858 668,990 562,428        
Stock awards granted (in shares or units)     320,284 290,956 340,660 56,883 100,645    
Forfeited (in shares)     (18,058) (13,986) (5,081)     (58,105)  
Vested (in shares)     (323,706) (276,102) (229,017)        
Unvested at the end of the period (in shares)     648,378 669,858 668,990        
Restricted shares expected to vest (in shares)     631,169            
Weighted Average Grant Date Fair Value                  
Unvested at the beginning of the period (in dollars per share)     $ 32.77 $ 30.43 $ 35.69        
Grant date fair value (in dollars per share)     $ 33.68 $ 37.74 $ 25.30     $ 49.15 $ 53.31
Forfeited (in dollars per share)     $ 34.23 $ 34.38 $ 29.83        
Vested (in dollars per share)     $ 32.86 $ 32.24 $ 35.74        
Unvested at the end of the period (in dollars per share)     $ 33.13 $ 32.77 $ 30.43        
Restricted shares expected to vest (in dollars per share)     $ 33.13            
Other share-based compensation disclosures                  
Aggregate intrinsic value of restricted shares that vested     $ 11.2 $ 10.3 $ 5.9        
Voluntarily cancelled (in shares)     18,058 13,986 5,081     58,105  
Non-cash compensation charge recognized in connection with cancellations               $ 1.2  
Performance period of the award (in years)               3 years 3 years
Potential earned PSUs payout for defined levels of performance under awards                  
Earned PSUs payout (as a percent of PSUs granted) on 75th or greater percentile rank               200.00%  
Earned PSUs payout (as a percent of PSUs granted) on 50th percentile rank               100.00%  
Earned PSUs payout (as a percent of PSUs granted) on 25th percentile rank               50.00%  
Performance share units granted on percentile rank below 25th (as a percent)               0.00%  
The number of percentile ranks to fall between to earn interpolated PSUs, conditioned on the percentile rank exceeding 25%               2  
Assumptions used to value stock awards                  
Grant date fair value (in dollars per share)     $ 33.68 $ 37.74 $ 25.30     $ 49.15 $ 53.31
Baseline value per common share (in dollars per share)               $ 35.17 $ 37.84
Expected volatility of common shares (as a percent)               61.10% 62.20%
Risk-free interest rate (as a percent)               1.32% 1.38%