Redeemable Noncontrolling Interests (Narrative) (Details)
 3 Months Ended 6 Months Ended 12 Months Ended
Jun. 30, 2019
USD ($)
Jun. 26, 2019
USD ($)
property
Mar. 10, 2017
USD ($)
Feb. 28, 2017
USD ($)
$ / shares
shares
Feb. 27, 2017
USD ($)
Feb. 03, 2017
USD ($)
$ / shares
shares
Jun. 30, 2019
USD ($)
Mar. 31, 2019
USD ($)
Jun. 30, 2018
USD ($)
Jun. 30, 2019
USD ($)
Jun. 30, 2018
USD ($)
Dec. 31, 2018
USD ($)
Apr. 30, 2017
shares
Redeemable Noncontrolling Interest [Line Items]                          
General and administrative             $ 16,427,000   $ 13,455,000 $ 29,020,000 $ 29,540,000    
Expiration period                   10 years      
Minimum [Member]                          
Redeemable Noncontrolling Interest [Line Items]                          
Percentage of interest in venture 20.00%           20.00%     20.00%      
Maximum [Member]                          
Redeemable Noncontrolling Interest [Line Items]                          
Percentage of interest in venture 85.00%           85.00%     85.00%      
Mack-Cali Realty LP [Member]                          
Redeemable Noncontrolling Interest [Line Items]                          
General and administrative             $ 16,427,000   $ 13,455,000 $ 29,020,000 $ 29,540,000    
Rockpoint [Member] | Cash Flow From Operations [Member]                          
Redeemable Noncontrolling Interest [Line Items]                          
Invested capital $ 400,000,000           400,000,000     400,000,000      
Rockpoint [Member] | Cash Flow From Capital Events, Distribution Five [Member]                          
Redeemable Noncontrolling Interest [Line Items]                          
Invested capital 400,000,000           400,000,000     400,000,000      
Rockpoint [Member] | Cash Flow From Capital Events, Distribution Six [Member]                          
Redeemable Noncontrolling Interest [Line Items]                          
Invested capital 400,000,000           400,000,000     $ 400,000,000      
Rockpoint [Member] | Distribution One [Member]                          
Redeemable Noncontrolling Interest [Line Items]                          
Purchase price   $ 173,500,000                      
Purchase price, less distributions   198,500,000                      
Rockpoint [Member] | Distribution Two [Member]                          
Redeemable Noncontrolling Interest [Line Items]                          
Purchase price, less distributions   1,500,000                      
Series A Units [Member] | Mack-Cali Realty LP [Member]                          
Redeemable Noncontrolling Interest [Line Items]                          
Preferred units shares issued | shares           42,800              
Preferred unit annual rate           3.50%              
Percentage of interest in venture           37.50%              
Preferred unit in operating partnership           $ 1,000              
Convertible preferred units | shares           28.15              
Expiration period           5 years              
Shares that may be converted to common units | shares           1,204,820              
Common unit distribution per unit declared | $ / shares           $ 35.52              
Series A-1 Units [Member] | Mack-Cali Realty LP [Member]                          
Redeemable Noncontrolling Interest [Line Items]                          
Preferred units shares issued | shares       9,213                 91
Preferred unit annual rate       3.50%                  
Percentage of interest in venture       13.80%                  
Preferred unit in operating partnership       $ 1,000                  
Convertible preferred units | shares       27.936                  
Expiration period       5 years           5 years      
Shares that may be converted to common units | shares       257,375                  
Common unit distribution per unit declared | $ / shares       $ 35.80                  
Preferred Units [Member] | Rockpoint [Member]                          
Redeemable Noncontrolling Interest [Line Items]                          
Pro rata distribution                   10.947%      
Preferred Units [Member] | Rockpoint [Member] | Cash Flow From Operations [Member]                          
Redeemable Noncontrolling Interest [Line Items]                          
Pro rata distribution                   21.89%      
Preferred Units [Member] | Rockpoint [Member] | Cash Flow From Capital Events, Distribution Five [Member]                          
Redeemable Noncontrolling Interest [Line Items]                          
Pro rata distribution                   21.89%      
Preferred Units [Member] | RRT [Member] | Cash Flow From Operations [Member]                          
Redeemable Noncontrolling Interest [Line Items]                          
Pro rata distribution                   2.65%      
Preferred Units [Member] | RRT [Member] | Cash Flow From Capital Events, Distribution Five [Member]                          
Redeemable Noncontrolling Interest [Line Items]                          
Pro rata distribution                   2.65%      
Preferred Units [Member] | RRT [Member] | Cash Flow From Capital Events, Distribution Six [Member]                          
Redeemable Noncontrolling Interest [Line Items]                          
Pro rata distribution                   1.325%      
Monaco [Member] | Series A-1 Units [Member] | Mack-Cali Realty LP [Member]                          
Redeemable Noncontrolling Interest [Line Items]                          
Preferred units shares issued | shares       9,122                  
Common Units [Member] | RRT [Member] | Cash Flow From Operations [Member]                          
Redeemable Noncontrolling Interest [Line Items]                          
Pro rata distribution                   75.46%      
Common Units [Member] | RRT [Member] | Cash Flow From Capital Events, Distribution Five [Member]                          
Redeemable Noncontrolling Interest [Line Items]                          
Pro rata distribution                   75.46%      
Common Units [Member] | RRT [Member] | Cash Flow From Capital Events, Distribution Six [Member]                          
Redeemable Noncontrolling Interest [Line Items]                          
Pro rata distribution                   87.728%      
Investment Agreement [Member] | Rockpoint [Member]                          
Redeemable Noncontrolling Interest [Line Items]                          
Contributed equity value     $ 1,230,000,000                    
Incremental closing payments, Limited Partnership interest   46,000,000 $ 150,000,000         $ 45,000,000       $ 105,000,000  
Investment Agreement [Member] | Rockpoint [Member] | Minimum [Member]                          
Redeemable Noncontrolling Interest [Line Items]                          
Incremental closing payments, Limited Partnership interest                       $ 105,000,000  
Investment Agreement [Member] | Rockpoint [Member] | Maximum [Member]                          
Redeemable Noncontrolling Interest [Line Items]                          
Contributed amount to obtain equity units         $ 300,000,000                
Contributed amount to obtain equity units         $ 300,000,000     $ 300,000,000          
Add On Investment Agreement [Member] | Rockpoint [Member]                          
Redeemable Noncontrolling Interest [Line Items]                          
Contributed amount to obtain equity units   $ 100,000,000                      
Number of properties in which additional interest was acquired during period | property   2                      
Right of first refusal to invest 100,000,000                        
General and administrative             371,000,000,000     $ 371,000,000,000      
Add On Investment Agreement [Member] | Rockpoint [Member] | Maximum [Member]                          
Redeemable Noncontrolling Interest [Line Items]                          
Contributed amount to obtain equity units 154,000,000                        
RRLP [Member] | Cash Flow From Operations [Member]                          
Redeemable Noncontrolling Interest [Line Items]                          
Annual return on the equity value                   6.00%      
RRLP [Member] | Rockpoint [Member] | Cash Flow From Operations [Member]                          
Redeemable Noncontrolling Interest [Line Items]                          
Annual return                   6.00%      
Base return                   4.64%      
RRLP [Member] | Rockpoint [Member] | Cash Flow From Capital Events, Distribution Three [Member]                          
Redeemable Noncontrolling Interest [Line Items]                          
Base return                   4.64%      
RRLP [Member] | Rockpoint [Member] | Cash Flow From Capital Events, Distribution Four [Member]                          
Redeemable Noncontrolling Interest [Line Items]                          
Capital base return                   4.64%      
RRLP [Member] | Rockpoint [Member] | Cash Flow From Capital Events, Distribution Six [Member]                          
Redeemable Noncontrolling Interest [Line Items]                          
Pro rata share                   50.00%      
RRLP [Member] | RRT [Member]                          
Redeemable Noncontrolling Interest [Line Items]                          
Loan-to-value ratio   65.00%                      
Equity capitalization percent                   10.00%      
RRLP [Member] | RRT [Member] | Cash Flow From Operations [Member]                          
Redeemable Noncontrolling Interest [Line Items]                          
Base return                   95.36%      
RRLP [Member] | RRT [Member] | Cash Flow From Capital Events, Distribution Three [Member]                          
Redeemable Noncontrolling Interest [Line Items]                          
Base return                   95.36%      
RRLP [Member] | RRT [Member] | Cash Flow From Capital Events, Distribution Four [Member]                          
Redeemable Noncontrolling Interest [Line Items]                          
Capital base return                   95.36%      
RRLP [Member] | Investment Agreement [Member] | Rockpoint [Member] | Cash Flow From Operations [Member]                          
Redeemable Noncontrolling Interest [Line Items]                          
Base return               5.00%          
RRLP [Member] | Investment Agreement [Member] | Rockpoint [Member] | Cash Flow From Capital Events, Distribution Three [Member]                          
Redeemable Noncontrolling Interest [Line Items]                          
Base return               95.00%          
RRLP [Member] | Investment Agreement [Member] | Rockpoint [Member] | Cash Flow From Capital Events, Distribution Four [Member]                          
Redeemable Noncontrolling Interest [Line Items]                          
Capital base return               5.00%          
RRLP [Member] | Investment Agreement [Member] | RRT [Member] | Cash Flow From Operations [Member]                          
Redeemable Noncontrolling Interest [Line Items]                          
Base return               95.00%          
RRLP [Member] | Investment Agreement [Member] | RRT [Member] | Cash Flow From Capital Events, Distribution Three [Member]                          
Redeemable Noncontrolling Interest [Line Items]                          
Base return               5.00%          
RRLP [Member] | Investment Agreement [Member] | RRT [Member] | Cash Flow From Capital Events, Distribution Four [Member]                          
Redeemable Noncontrolling Interest [Line Items]                          
Capital base return               95.00%          
RRLP [Member] | Credit Enhancement Note [Member]                          
Redeemable Noncontrolling Interest [Line Items]                          
Maximum borrowing capacity 50,000,000           50,000,000     $ 50,000,000      
Increased line of credit $ 25,000,000           $ 25,000,000     $ 25,000,000