Quarterly report pursuant to Section 13 or 15(d)

Share-Based Compensation (Details)

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Share-Based Compensation (Details) (USD $)
In Millions, except Share data, unless otherwise specified
6 Months Ended
Jun. 30, 2011
Jun. 30, 2010
Options    
Options exercised to purchase common shares ("options") (in shares) 180,464 175,443
Performance share units
   
Share-Based Compensation    
Stock awards granted (in shares or units) 56,883  
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%  
Assumptions used to value stock awards    
Baseline value per common share (in dollars per share) $ 35.17  
Expected volatility for common shares (as a percent) 61.10%  
Risk-free interest rate (as a percent) 1.32%  
Other Share-Based Compensation Disclosures    
Performance period of the award (in years) three years  
Grant date fair value (in dollars per share) $ 49.15  
The number of percentile ranks to fall between to earn interpolated PSUs, conditioned on the percentile rank exceeding 25% 2  
Restricted shares
   
Share-Based Compensation    
Stock awards granted (in shares or units) 280,134  
Other Share-Based Compensation Disclosures    
Grant date fair value (in dollars per share) $ 35.30  
Vesting period of awards to non employee Trustees first anniversary of the grant date  
Restricted common shares, forfeiture restrictions lapsed (in shares) 304,281  
Weighted average grant date fair value of restricted common share, forfeiture restrictions lapsed (in dollars per share) $ 32.56  
Aggregate intrinsic value of restricted common shares, forfeiture restrictions lapsed $ 10.8  
Options
   
Options    
Options exercised to purchase common shares ("options") (in shares) 180,464  
Weighted average exercise price of options (in dollars per share) $ 12.74  
Aggregate intrinsic value of options exercised $ 3.9