Recent Transactions (Schedule Of Properties Acquired) (Details)
 12 Months Ended
Dec. 31, 2017
USD ($)
ft²
item
Dec. 31, 2016
USD ($)
ft²
item
Business Acquisition [Line Items]    
Number of Buildings, Acquired | item 9 5
Rentable Square Feet, Acquired | ft² 1,392,500 1,058,462
Acquisition Cost $ 394,589,000 $ 326,811,000
Loan balance $ 2,809,568,000 $ 2,340,009,000
11 Martine Avenue [Member]    
Business Acquisition [Line Items]    
Number of Buildings, Acquired | item [1]   1
Rentable Square Feet, Acquired | ft² [1]   82,000
Acquisition Cost [1]   $ 10,750,000
Number of apartment units | item   4
Area of property (in square feet) | ft²   262,000
320, 321 University Avenue [Member]    
Business Acquisition [Line Items]    
Number of Buildings, Acquired | item [2]   2
Rentable Square Feet, Acquired | ft² [2]   147,406
Acquisition Cost [2]   $ 23,000,000
101 Wood Avenue South [Member]    
Business Acquisition [Line Items]    
Number of Buildings, Acquired | item [3]   1
Rentable Square Feet, Acquired | ft² [3]   262,841
Acquisition Cost [3]   $ 82,300,000
111 River Street [Member]    
Business Acquisition [Line Items]    
Number of Buildings, Acquired | item [3]   1
Rentable Square Feet, Acquired | ft² [3]   566,215
Acquisition Cost [3]   $ 210,761,000
Red Bank Portfolio [Member]    
Business Acquisition [Line Items]    
Number of Buildings, Acquired | item [4] 3  
Rentable Square Feet, Acquired | ft² [4] 279,472  
Acquisition Cost [4] $ 27,228,000  
Short Hills / Madison Portfolio [Member]    
Business Acquisition [Line Items]    
Number of Buildings, Acquired | item [5] 6  
Rentable Square Feet, Acquired | ft² [5] 1,113,028  
Acquisition Cost [5] $ 367,361,000  
Unsecured Revolving Credit Facility [Member] | Short Hills / Madison Portfolio [Member]    
Business Acquisition [Line Items]    
Loan balance $ 124,500,000  
[1] Acquisition represented four units of condominium interests which collectively comprise floors 2 through 5. Upon completion of the acquisition, the Company owns the entire 14-story 262,000 square-foot building. The acquisition was funded using available cash.
[2] This acquisition was funded through borrowings under the Company's unsecured revolving credit facility.
[3] This acquisition was funded using available cash and through borrowings under the Company's unsecured revolving credit facility.
[4] This acquisition was funded through borrowings under the Company's unsecured revolving credit facility.
[5] This acquisition was funded through borrowings under the Company's unsecured revolving credit facility and a new $124.5 million loan secured by three of the properties.