Quarterly report pursuant to Section 13 or 15(d)

Mack-Cali Realty Corporation Stockholders' Equity And Mack-Cali Realty, L.P.'s Partners' Capital (Stock Option Plans) (Narrative) (Details)

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Mack-Cali Realty Corporation Stockholders' Equity And Mack-Cali Realty, L.P.'s Partners' Capital (Stock Option Plans) (Narrative) (Details)
 3 Months Ended 9 Months Ended 12 Months Ended
Jul. 26, 2020
$ / shares
shares
Jun. 05, 2015
item
$ / shares
shares
Sep. 30, 2020
USD ($)
shares
Sep. 30, 2019
USD ($)
Sep. 30, 2020
USD ($)
$ / shares
shares
Sep. 30, 2019
USD ($)
shares
Dec. 31, 2019
Jul. 25, 2020
shares
May 31, 2013
shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                  
Weighted average remaining contractual life         3 years 10 months 24 days   5 years 4 months 24 days    
Share price | $ / shares   $ 17.31              
Options exercised         0 0      
Stock options expense | $     $ 192,000 $ 0 $ 192,000 $ 0      
Shares Under Options - Granted   800,000 172,495            
Common stock trade share price | $ / shares   $ 25.00              
Annual installments | item   3              
Exercisable period         10 years        
Common stock trading days         30 days        
Share price - weighted average fair value of options granted | $ / shares         $ 2.95        
Three Equal Annual Installment [Member]                  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                  
Shares Under Options - Granted   400,000              
Common Stock Trades [Member]                  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                  
Shares Under Options - Granted   400,000              
2013 Incentive Stock Plan [Member]                  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                  
Reserved stocks for issuance                 4,600,000
Shares Under Options - Granted         172,495        
MAG Partners [Member]                  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                  
Shares Under Options - Granted 157,505                
MAG Partners [Member] | Maximum [Member] | Common Class At $14.39 [Member]                  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                  
Number of shares authorized               230,000  
Share price | $ / shares $ 14.39                
MAG Partners [Member] | Maximum [Member] | Common Class At $20.00 [Member]                  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                  
Share price | $ / shares $ 20.00                
Number of shares authorized 100,000                
Share price | $ / shares $ 20.00