Significant Accounting Policies (Narrative) (Details) - USD ($)
 3 Months Ended 9 Months Ended 12 Months Ended
Jan. 01, 2019
Sep. 30, 2020
Dec. 31, 2019
Sep. 30, 2019
Sep. 30, 2020
Sep. 30, 2019
Dec. 31, 2017
Jun. 30, 2020
Jan. 03, 2020
Jun. 30, 2019
Dec. 31, 2018
Significant Accounting Policies [Line Items]                      
Capitalized development and construction salaries and other related costs   $ 400,000   $ 500,000 $ 1,300,000 $ 1,600,000          
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   1,074,000   1,121,000 3,158,000 3,478,000          
Write-off of unamortized deferred financing costs   0   285,000 0 285,000          
Leasing personnel costs   2,128,000   $ 534,000 3,829,000 $ 1,818,000          
Deferred tax asset   $ 18,500,000     18,500,000   $ 5,300,000        
Income taxes, material adjustment amount         $ 0            
Common stock, shares outstanding   90,712,055 90,595,176   90,712,055       90,595,197    
Common units outstanding   9,672,558 9,612,064 9,973,344 9,672,558 9,973,344   9,586,528 9,488,794 9,976,344 10,229,349
LTIP units outstanding   1,738,065 1,826,331 1,826,331 1,738,065 1,826,331   2,659,518 1,949,601 1,826,331 1,707,106
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                
Stock compensation expense   $ 723,000   $ 1,980,000 $ 5,692,000 $ 6,051,000          
Distributions payable, pay date     Jan. 10, 2020                
Increase (decrease) to valuation allowance             $ (5,300,000)        
Federal income tax rate         21.00%            
Gain from extinguishment of debt, net   0   (98,000) $ 0 1,801,000          
Net income (loss)   (41,118,000)   (56,021,000) (111,896,000) 199,244,000          
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 from extinguishment of debt, net       (98,000)   1,801,000          
Net income (loss)   $ (41,118,000)   $ (56,021,000) $ (111,896,000) $ 199,244,000          
LTIP value     $ 400,000                
Dividends paid     18,100,000                
Common Units [Member] | Mack-Cali Realty LP [Member] | Limited Partner Common Unitholders [Member]                      
Significant Accounting Policies [Line Items]                      
Dividends paid     $ 1,900,000