Significant Accounting Policies (Narrative) (Details) - USD ($)
 3 Months Ended 12 Months Ended
Jan. 01, 2019
Dec. 31, 2020
Sep. 30, 2020
Jun. 30, 2020
Mar. 31, 2020
Dec. 31, 2019
Sep. 30, 2019
Jun. 30, 2019
Mar. 31, 2019
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Jan. 03, 2020
Significant Accounting Policies [Line Items]                            
Capitalized development and construction salaries and other related costs                   $ 2,000,000.0 $ 2,100,000 $ 2,300,000    
Maximum period after cessation of major construction activity that projects are considered complete                   1 year        
Threshold of investment value for discontinuation of equity method accounting   $ 0               $ 0        
Amortization of deferred financing costs                   4,625,000 4,625,000 5,028,000    
Losses on extinguishment of debt, including discontinued operations                   1,800,000 1,800,000 1,800,000    
Gain (Loss) from extinguishment of debt, net                   (272,000) 1,648,000 (10,750,000)    
Deferred leasing costs                       $ 3,463,000    
Leasing personnel costs                   1,483,000 $ 2,261,000      
Goodwill impairment                   0        
Difference between the estimated net basis and net assets of the rental property for federal income tax purposes   669,239,000               669,239,000        
Deferred tax asset   $ 16,200,000               16,200,000     $ 5,300,000  
Income taxes, material adjustment amount                   $ 0        
Common stock, shares outstanding   90,712,417       90,595,176       90,712,417 90,595,176     90,595,197
Common units outstanding                           9,488,794
LTIP units outstanding                           1,949,601
Distributions payable, record date           Jan. 03, 2020                
Distributions payable, approved date           Dec. 17, 2019                
Common stock dividends and common unit distributions per share           $ 0.20                
Dividends paid per common share                   $ 0.60 $ 0.80 $ 0.80    
Dividends paid, percent representing ordinary income                   19.00%        
Dividends paid, percent representing return of capital to shareholders                       47.00%    
Dividends paid, percent representing capital gain                   81.00% 100.00% 53.00%    
Stock compensation expense                   $ 7,635,000 $ 8,161,000 $ 6,894,000    
Distributions payable, pay date           Jan. 10, 2020                
Increase (decrease) to valuation allowance                         $ (5,300,000)  
Federal income tax rate                   21.00%        
Write off of unamortized deferred financing costs                   $ 0 400,000 600,000    
Net income (loss)   $ 78,298,000 $ (41,118,000) $ (32,934,000) $ (37,844,000) $ (55,408,000) $ (56,021,000) $ (20,329,000) $ 275,594,000 (33,598,000) 143,836,000 106,401,000    
Accounting Standards Update 2017-12 [Member]                            
Significant Accounting Policies [Line Items]                            
Net income (loss) $ 400,000                          
Mack-Cali Realty LP [Member]                            
Significant Accounting Policies [Line Items]                            
Gain (Loss) from extinguishment of debt, net                   (272,000) 1,648,000 (10,750,000)    
Taxable income                   $ 79,293,000 $ 71,151,000 $ 82,106,000    
Common units outstanding   9,649,031       9,612,064       9,649,031 9,612,064 10,229,349 10,438,855  
LTIP units outstanding   1,722,929       1,826,331       1,722,929 1,826,331 1,707,106 1,230,877  
Net income (loss)   $ 78,298,000 $ (41,118,000) $ (32,934,000) $ (37,844,000) $ (55,408,000) $ (56,021,000) $ (20,329,000) $ 275,594,000 $ (33,598,000) $ 143,836,000 $ 106,401,000    
Dividends paid           400,000                
Mack-Cali Realty LP [Member] | General Partner Common Unitholders [Member]                            
Significant Accounting Policies [Line Items]                            
Dividends paid           18,100,000                
Mack-Cali Realty LP [Member] | Limited Partner Common Unitholders [Member]                            
Significant Accounting Policies [Line Items]                            
Dividends paid           $ 1,900,000