Recent Transactions (Rockpoint Transaction) (Narrative) (Details)
$ in Thousands
 3 Months Ended 12 Months Ended
Jun. 30, 2019
USD ($)
Jun. 26, 2019
USD ($)
property
Mar. 10, 2017
USD ($)
Feb. 27, 2017
USD ($)
Jun. 26, 2016
USD ($)
Mar. 31, 2019
USD ($)
Dec. 31, 2020
USD ($)
Dec. 31, 2019
USD ($)
Dec. 31, 2018
USD ($)
Preferred Units [Line Items]                  
Payment for borrowings             $ 516,000 $ 617,000 $ 494,000
Assets             5,147,786 5,292,798  
RRLP [Member]                  
Preferred Units [Line Items]                  
Mortgages and loans payable             1,700,000 1,400  
Other assets             221,100 273,300  
Other liabilities             90,400 115,200  
Assets             $ 3,000,000 $ 2,800,000  
Investment Agreement [Member] | Rockpoint [Member]                  
Preferred Units [Line Items]                  
Contributed amount to obtain equity units           $ 300,000      
Incremental closing payments, Limited Partnership interest   $ 46,000 $ 150,000   $ 46,000 45,000     $ 105,000
Contributed equity value     $ 1,230,000            
Investment Agreement [Member] | Rockpoint [Member] | Maximum [Member]                  
Preferred Units [Line Items]                  
Contributed amount to obtain equity units       $ 300,000   $ 300,000      
Preferred Units, Contribution To Obtain Equity Units       $ 300,000          
Add On Investment Agreement [Member] | Rockpoint [Member]                  
Preferred Units [Line Items]                  
Contributed amount to obtain equity units   $ 100,000              
Number of properties in which additional interest was acquired during period | property   2              
Payment for borrowings $ 100,000                
Right of first refusal to invest 100,000                
Add On Investment Agreement [Member] | Rockpoint [Member] | Maximum [Member]                  
Preferred Units [Line Items]                  
Preferred Units, Contribution To Obtain Equity Units $ 154,000