Recent Transactions (Unconsolidated Joint Venture Activity) (Narrative) (Details)
$ in Thousands
 1 Months Ended 3 Months Ended 12 Months Ended
Sep. 29, 2017
USD ($)
Sep. 21, 2017
USD ($)
Feb. 15, 2017
USD ($)
Jan. 31, 2017
USD ($)
May 26, 2016
USD ($)
item
Apr. 01, 2016
Oct. 31, 2016
Mar. 31, 2016
Dec. 31, 2017
USD ($)
item
Dec. 31, 2016
USD ($)
Dec. 31, 2015
USD ($)
Real Estate Properties [Line Items]                      
Gain (loss) on sale of investment in unconsolidated joint venture                 $ 23,131 $ 5,670 $ 6,448
Proceeds from the sale of investments in unconsolidated joint ventures                 110,269 6,420 6,448
Investment in unconsolidated joint ventures                 36,060 $ 35,930 $ 78,027
Debt                 $ 2,809,568    
Unconsolidated Joint Venture Multi-Family Properties [Member]                      
Real Estate Properties [Line Items]                      
Number of units | item                 3,275    
Multi-Family Properties [Member]                      
Real Estate Properties [Line Items]                      
Number of units | item                 5,826    
PruRose Riverwalk G, L.L.C. [Member]                      
Real Estate Properties [Line Items]                      
Percent of pari passu interest             22.50%        
Percentage of additional interest acquired           50.00%   25.00%      
Number of units | item                 316    
PruRose Port Imperial South 15, L.L.C. [Member]                      
Real Estate Properties [Line Items]                      
Number of units | item         236            
PruRose Port Imperial South 13, L.L.C. [Member] | Multi-Family Properties [Member]                      
Real Estate Properties [Line Items]                      
Number of units | item         280            
Pru Rose Port Imperial South 15 L.L.C. And Pru Rose Port Imperial South 13 L.L.C. [Member]                      
Real Estate Properties [Line Items]                      
Gain (loss) on sale of investment in unconsolidated joint venture         $ 5,700            
Proceeds from the sale of investments in unconsolidated joint ventures         $ 6,400            
KPG-P 100 IMW JV, LLC, Keystone-Penn And Keystone-Tristate [Member]                      
Real Estate Properties [Line Items]                      
Gain (loss) on sale of investment in unconsolidated joint venture       $ 7,400              
Proceeds from the sale of investments in unconsolidated joint ventures       $ 9,700              
Elmajo Urban Renewal Associates, LLC And Estuary Urban Renewal Associates, LLC [Member] | Unconsolidated Joint Venture Multi-Family Properties [Member]                      
Real Estate Properties [Line Items]                      
Gain (loss) on sale of investment in unconsolidated joint venture     $ 5,100                
Proceeds from the sale of investments in unconsolidated joint ventures     $ 5,100                
Percentage of interest in venture     7.50%                
RoseGarden Monaco, L.L.C. [Member]                      
Real Estate Properties [Line Items]                      
Gain (loss) on sale of investment in unconsolidated joint venture   $ (1,400)                  
RoseGarden Monaco, L.L.C. [Member] | Unconsolidated Joint Venture Multi-Family Properties [Member]                      
Real Estate Properties [Line Items]                      
Percentage of interest in venture [1],[2]                 41.67%    
Number of units | item [2]                 250    
KPG-MCG Curtis JV, LLC [Member]                      
Real Estate Properties [Line Items]                      
Gain (loss) on sale of investment in unconsolidated joint venture $ 12,000                    
Proceeds from the sale of investments in unconsolidated joint ventures 102,500                    
Net sales proceeds held by qualified intermediary                 $ 5,600    
Debt $ 75,000                    
[1] Company's effective ownership % represents the Company's entitlement to residual distributions after payments of priority returns, where applicable.
[2] On September 21, 2017, the joint venture agreement was terminated. Accordingly, the Company wrote off the carrying value of its investment in the joint venture and recorded a loss of $1.4 million on the disposition of its joint venture interest.