Significant Accounting Policies (Narrative) (Details) - USD ($)
 3 Months Ended 6 Months Ended
Jun. 30, 2018
Dec. 31, 2017
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Jul. 03, 2018
Jan. 03, 2018
Significant Accounting Policies [Line Items]              
Capitalized development and construction salaries and other related costs $ 600,000   $ 600,000 $ 1,200,000 $ 1,200,000    
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 1,145,000   1,175,000 2,241,000 2,278,000    
Loss from extinguishment of debt, net       (10,289,000) (239,000)    
Write-off of unamortized deferred financing costs       105,000 0    
Deferred leasing costs 935,000   946,000 1,628,000 1,988,000    
Deferred tax asset $ 10,100,000     10,100,000      
Income taxes, material adjustment amount       $ 0      
Common stock, shares outstanding 90,286,268 89,914,113   90,286,268     89,914,658
Common units outstanding 10,211,079 10,438,855   10,211,079     10,438,855
LTIP units outstanding 1,850,811 1,230,877   1,850,811     1,230,877
Distributions payable, record date   Jan. 03, 2018   Jul. 03, 2018      
Distributions payable, approved date   Dec. 12, 2017   Jun. 13, 2018      
Common stock dividends and common unit distributions per share   $ 0.20   $ 0.20      
Restricted stock expense $ 657,000   $ 2,046,000 $ 3,189,000 $ 3,099,000    
Distributions payable, pay date   Jan. 12, 2018   Jul. 13, 2018      
Federal income tax rate       21.00%      
Subsequent Event [Member]              
Significant Accounting Policies [Line Items]              
Common stock, shares outstanding           90,288,634  
Common units outstanding           10,208,785  
LTIP units outstanding           1,850,811