| 
           Shareholders' Equity (Details) (USD $) 
         | 
        3 Months Ended | |
|---|---|---|
| 
           Mar. 31, 2012 
         | 
        
           Mar. 31, 2011 
         | 
      |
| Common Shares | ||
| Number of common shares for each converted common unit (in shares) | 34,550 | |
| Dividends declared per common share (in dollars per share) | $ 0.275 | $ 0.4125 | 
| X | ||||||||||
| 
             
- Definition 
          Represents the amount of common shares issued upon conversion of each common unit. No definition available. 
  | 
        
| X | ||||||||||
| 
             
- Definition 
          Aggregate dividends declared during the period for each share of common stock outstanding. Reference 1: http://www.xbrl.org/2003/role/presentationRef 
  | 
        
| X | ||||||||||
| 
             
- Details 
          
  |