Significant Accounting Policies (Narrative) (Details) - USD ($)
 3 Months Ended 9 Months Ended
Sep. 30, 2018
Dec. 31, 2017
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Oct. 02, 2018
Jan. 03, 2018
Significant Accounting Policies [Line Items]              
Capitalized development and construction salaries and other related costs $ 600,000   $ 600,000 $ 1,700,000 $ 1,800,000    
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 1,302,000   1,184,000 3,543,000 3,462,000    
Loss from extinguishment of debt, net       (10,289,000) (239,000)    
Write-off of unamortized deferred financing costs       105,000 0    
Deferred leasing costs 598,000   607,000 2,226,000 2,595,000    
Deferred tax asset $ 9,800,000     9,800,000      
Income taxes, material adjustment amount       $ 0      
Common stock, shares outstanding 90,307,280 89,914,113   90,307,280     89,914,658
Common units outstanding 10,186,785 10,438,855   10,186,785     10,438,855
LTIP units outstanding 1,821,384 1,230,877   1,821,384     1,230,877
Distributions payable, record date   Jan. 03, 2018   Oct. 02, 2018      
Distributions payable, approved date   Dec. 12, 2017   Sep. 12, 2018      
Common stock dividends and common unit distributions per share   $ 0.20   $ 0.20      
Restricted stock expense $ 1,770,000   $ 2,169,000 $ 4,959,000 $ 5,268,000    
Distributions payable, pay date   Jan. 12, 2018   Oct. 12, 2018      
Federal income tax rate       21.00%      
Subsequent Event [Member]              
Significant Accounting Policies [Line Items]              
Common stock, shares outstanding           90,307,376  
Common units outstanding           10,186,785  
LTIP units outstanding           1,821,384