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 ($)
$ in Thousands
3 Months Ended 6 Months Ended
Feb. 15, 2017
Jan. 31, 2017
Jun. 30, 2016
Jun. 30, 2017
Jun. 30, 2016
Real Estate Properties [Line Items]          
Gain on sale of investment in unconsolidated joint venture     $ 5,670 $ 12,563 $ 5,670
Proceeds from the sale of investments in unconsolidated joint ventures       14,849 6,420
Investment in unconsolidated joint ventures       11,422 23,657
Mack-Cali Realty LP [Member]          
Real Estate Properties [Line Items]          
Gain on sale of investment in unconsolidated joint venture     $ 5,670 12,563 5,670
Proceeds from the sale of investments in unconsolidated joint ventures       14,849 6,420
Investment in unconsolidated joint ventures       $ 11,422 $ 23,657
KPG-P 100 IMW JV, LLC, Keystone-Penn And Keystone-Tristate [Member]          
Real Estate Properties [Line Items]          
Gain 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]          
Real Estate Properties [Line Items]          
Proceeds from the sale of investments in unconsolidated joint ventures $ 5,100        
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 on sale of investment in unconsolidated joint venture 5,100        
Proceeds from the sale of investments in unconsolidated joint ventures $ 5,100