Quarterly report pursuant to Section 13 or 15(d)

Recent Transactions (Unconsolidated Joint Venture Activity) (Narrative) (Details)

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Recent Transactions (Unconsolidated Joint Venture Activity) (Narrative) (Details) - USD ($)
 3 Months Ended 9 Months Ended
Sep. 29, 2017
Sep. 21, 2017
Feb. 15, 2017
Jan. 31, 2017
Sep. 30, 2017
Sep. 30, 2017
Sep. 30, 2016
Dec. 31, 2016
Real Estate Properties [Line Items]                
Gain (loss) on sale of investment in unconsolidated joint venture         $ 10,568,000 $ 23,131,000 $ 5,670,000  
Proceeds from the sale of investments in unconsolidated joint ventures           107,009,000 6,420,000  
Investment in unconsolidated joint ventures           19,279,000 31,318,000  
Net sales proceeds held by qualified intermediary           45,300,000    
Debt         2,856,676,000 2,856,676,000   $ 2,357,055,000
Mack-Cali Realty LP [Member]                
Real Estate Properties [Line Items]                
Gain (loss) on sale of investment in unconsolidated joint venture         $ 10,568,000 23,131,000 5,670,000  
Proceeds from the sale of investments in unconsolidated joint ventures           107,009,000 6,420,000  
Investment in unconsolidated joint ventures           $ 19,279,000 $ 31,318,000  
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       $ 7,400,000        
Proceeds from the sale of investments in unconsolidated joint ventures       $ 9,700,000        
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     $ 5,100,000          
Proceeds from the sale of investments in unconsolidated joint ventures     $ 5,100,000          
RoseGarden Monaco, L.L.C. [Member]                
Real Estate Properties [Line Items]                
Gain (loss) on sale of investment in unconsolidated joint venture   $ (1,400,000)            
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% 41.67%    
KPG-MCG Curtis JV, LLC [Member]                
Real Estate Properties [Line Items]                
Gain (loss) on sale of investment in unconsolidated joint venture $ 12,000,000              
Proceeds from the sale of investments in unconsolidated joint ventures 102,500,000              
Net sales proceeds held by qualified intermediary 8,900,000              
Debt $ 75,000,000              
[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.