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Related Party Transactions (Details) (USD $)
6 Months Ended
Jun. 30, 2012
Jun. 30, 2011
Dec. 31, 2011
Treasury Stock Purchases from Related Party (Textual) [Abstract]      
Treasury stock, shares 1,213,560   1,134,528
Treasury stock $ 33,055,000   $ 31,120,000
Related Party Transactions (Additional Textual) [Abstract]      
Repurchase interests of partners and trust unit holders 47,000 169,000  
Board of Directors [Member]
     
Treasury Stock Purchases from Related Party (Textual) [Abstract]      
Treasury stock, shares 45,179    
Treasury stock $ 1,130,000