Significant Accounting Policies (Narrative) (Details) - USD ($)
 3 Months Ended 9 Months Ended 12 Months Ended
Sep. 30, 2016
Dec. 31, 2015
Sep. 30, 2015
Sep. 30, 2016
Sep. 30, 2015
Dec. 31, 2015
Oct. 05, 2016
Jan. 06, 2016
Significant Accounting Policies [Line Items]                
Capitalized development and construction salaries and other related costs $ 700,000   $ 900,000 $ 1,900,000 $ 3,500,000      
Construction, tenant improvement, and development in-progress 301,000,000 $ 88,700,000   $ 301,000,000   $ 88,700,000    
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 1,234,000   945,000 3,583,000 2,846,000      
Loss from extinguishment of debt, net       12,420,000        
Write off of unamortized deferred financing costs 346,000     346,000        
Deferred leasing costs 790,000   922,000 2,440,000 2,738,000      
Deferred tax asset $ 21,800,000     21,800,000        
Income taxes, material adjustment amount       $ 0        
Common stock, shares outstanding 89,647,337 89,583,950   89,647,337   89,583,950   89,584,008
Common units outstanding 10,497,946 10,516,844   10,497,946   10,516,844   10,516,844
LTIP units outstanding 657,373   657,373      
Distributions payable, record date       Oct. 05, 2016   Jan. 06, 2016    
Distributions payable, approved date   Dec. 08, 2015   Sep. 27, 2016        
Common stock dividends and common unit distributions per share   $ 0.15   $ 0.15        
Restricted stock expense $ 2,046,000   $ 695,000 $ 4,299,000 $ 1,500,000      
Distributions payable, pay date   Jan. 15, 2016   Oct. 14, 2016        
Subsequent Event [Member]                
Significant Accounting Policies [Line Items]                
Common stock, shares outstanding             89,647,443  
Common units outstanding             10,497,946  
LTIP units outstanding             657,373