Annual report pursuant to Section 13 and 15(d)

Significant Accounting Policies (Narrative) (Details)

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Significant Accounting Policies (Narrative) (Details) - USD ($)
 3 Months Ended 12 Months Ended
Jan. 01, 2019
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Jan. 03, 2020
Jan. 03, 2019
Dec. 31, 2016
Significant Accounting Policies [Line Items]                  
Capitalized development and construction salaries and other related costs       $ 2,100,000 $ 2,300,000 $ 2,200,000      
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 5,028,000 4,612,000      
Loss from extinguishment of debt, net       1,648,000 (10,750,000) (421,000)      
Losses on extinguishment of debt, including discontinued operations         1,800,000        
Deferred leasing costs       0 $ 3,463,000 3,146,000      
Goodwill impairment       0          
Difference between the estimated net basis and net assets of the rental property for federal income tax purposes   529,160,000   529,160,000          
Deferred tax asset   $ 9,800,000   9,800,000   $ (5,300,000)      
Income taxes, material adjustment amount       $ 0          
Common stock, shares outstanding   90,595,176 90,320,306 90,595,176 90,320,306     90,320,408  
Common units outstanding   9,612,064 10,229,349 9,612,064 10,229,349 10,438,855   10,174,285 10,488,105
LTIP units outstanding   1,826,331 1,707,106 1,826,331 1,707,106 1,230,877   1,762,170 657,373
Distributions payable, record date   Jan. 03, 2020 Jan. 03, 2019            
Distributions payable, approved date   Dec. 17, 2019 Dec. 11, 2018            
Common stock dividends and common unit distributions per share   $ 0.20 $ 0.20            
Dividends paid per common share       $ 0.80 $ 0.80 $ 0.70      
Dividends paid, percent representing ordinary income         47.00%        
Dividends paid, percent representing return of capital to shareholders           100.00%      
Dividends paid, percent representing capital gain       100.00%   53.00%      
Stock compensation expense       $ 8,161,000 $ 6,894,000 $ 7,447,000      
Distributions payable, pay date   Jan. 10, 2020 Jan. 11, 2019            
Increase (decrease) to valuation allowance           (5,300,000)      
Federal income tax rate       21.00%          
Write off of unamortized deferred financing costs       $ 400,000 600,000 400,000      
Gain (loss) from extinguishment of debt, net       1,648,000 (8,929,000) (421,000)      
Accounting Standards Update 2017-12 [Member]                  
Significant Accounting Policies [Line Items]                  
Adjustment to retained earnings $ 400,000                
Subsequent Event [Member]                  
Significant Accounting Policies [Line Items]                  
Common stock, shares outstanding             90,595,197    
Common units outstanding             9,488,794    
LTIP units outstanding             1,949,601    
Mack-Cali Realty LP [Member]                  
Significant Accounting Policies [Line Items]                  
Loss from extinguishment of debt, net       1,648,000 (10,750,000)        
Taxable income       71,151,000 82,106,000 97,037,000      
Gain (loss) from extinguishment of debt, net       $ 1,648,000 $ (8,929,000) $ (421,000)      
Dividends paid   $ 400,000 $ 400,000            
Mack-Cali Realty LP [Member] | General Partner Common Unitholders [Member]                  
Significant Accounting Policies [Line Items]                  
Dividends paid   18,100,000 18,100,000            
Mack-Cali Realty LP [Member] | Limited Partner Common Unitholders [Member]                  
Significant Accounting Policies [Line Items]                  
Dividends paid   $ 1,900,000 $ 2,000,000.0