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Real estate (Details 9) (USD $)
3 Months Ended 12 Months Ended
Mar. 31, 2012
Dec. 31, 2012
property
Dec. 31, 2011
Dec. 31, 2010
Real Estate Properties [Line Items]        
Real estate available and held for sale   $ 635,865,000 $ 677,458,000 $ 746,081,000
Number of properties transferred from held-for-sale to held-for-use   2    
Proceeds from sale of other real estate   319,300,000 154,000,000  
Income from sales of residential property 27,900,000 63,472,000 5,721,000 0
Carrying amount of assets transferred to held-for-use   49,800,000    
Residential Operating Properties
       
Real Estate Properties [Line Items]        
Real estate available and held for sale   374,100,000 419,000,000  
Land
       
Real Estate Properties [Line Items]        
Real estate available and held for sale   181,278,000 125,460,000  
Commercial Operating Properties
       
Real Estate Properties [Line Items]        
Real estate available and held for sale   $ 80,500,000 $ 133,000,000