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 1 Months Ended 12 Months Ended 0 Months Ended 12 Months Ended
Dec. 31, 2012
Stock Options
Dec. 31, 2012
Restricted shares
Dec. 31, 2011
Restricted shares
Dec. 31, 2010
Restricted shares
Dec. 31, 2012
Performance share units
Percentile_Rank
Dec. 31, 2011
Performance share units
Dec. 31, 2010
Performance share units
Mar. 31, 2010
Performance share units
2010 PSU Grants
Dec. 31, 2011
Performance share units
2010 PSU Grants
Mar. 31, 2011
Performance share units
2011 PSU Grants
Dec. 31, 2012
Performance share units
2011 PSU Grants
Mar. 02, 2012
Performance share units
2012 PSU Grants
Dec. 31, 2012
Performance share units
2012 PSU Grants
May 31, 2010
Maximum
Amended and Restated 2008 Omnibus Equity and Incentive Plan
Share-Based Compensation                            
Number of common shares of beneficial interest authroized to be issued                           5,900,000
Expiration period 10 years                          
Shares                            
Unvested at the beginning of the period (in shares)   648,378 669,858 668,990                    
Stock awards granted (in shares or units)   177,662 320,284 290,956       100,645   56,883   54,070    
Forfeited (in shares)   (17,019) (18,058) (13,986)         58,105          
Vested (in shares)   (374,378) (323,706) (276,102)                    
Unvested at the end of the period (in shares)   434,643 648,378 669,858                    
Restricted shares expected to vest (in shares)   419,014                        
Weighted Average Grant Date Fair Value                            
Unvested at the beginning of the period (in dollars per share)   $ 33.13 $ 32.77 $ 30.43                    
Grant date fair value (in dollars per share)   $ 23.64 $ 33.68 $ 37.74 $ 32.77 $ 49.15 $ 53.31              
Forfeited (in dollars per share)   $ 31.43 $ 34.23 $ 34.38                    
Vested (in dollars per share)   $ 32.72 $ 32.86 $ 32.24                    
Unvested at the end of the period (in dollars per share)   $ 29.67 $ 33.13 $ 32.77                    
Restricted shares expected to vest (in dollars per share)   $ 29.73                        
Other Share-based Compensation Additional Disclosures                            
Aggregate intrinsic value of restricted common shares, forfeiture restrictions lapsed   $ 9.0 $ 11.2 $ 10.3                    
Aggregate grant date fair value                 5.4   2.8   1.8  
Non-cash compensation charge recognized in connection with cancellations                 $ 1.2          
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 between such percentile ranks, conditioned on the percentile rank exceeding 25%         2                  
Performance period of the award         3 years                  
Assumptions used to value stock awards                            
Baseline value per common share (in dollars per share)         $ 24.39 $ 35.17 $ 37.84              
Expected volatility of common shares (as a percent)         43.20% 61.10% 62.20%              
Risk-free interest rate (as a percent)         0.41% 1.32% 1.38%