Significant Accounting Policies (Narrative) (Details) - USD ($)
 3 Months Ended 12 Months Ended
Dec. 31, 2016
Sep. 30, 2016
Jun. 30, 2016
Dec. 31, 2015
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Jan. 05, 2017
Jan. 06, 2016
Dec. 31, 2013
Significant Accounting Policies [Line Items]                    
Capitalized development and construction salaries and other related costs         $ 2,600,000 $ 4,200,000 $ 3,100,000      
Construction, tenant improvement, and development in-progress $ 361,100,000     $ 88,700,000 $ 361,100,000 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         4,582,000 3,790,000 3,274,000      
Loss from extinguishment of debt, net (23,658,000) $ (19,302,000) $ 12,420,000   12,420,000      
Write off of unamortized deferred financing costs         745,000 0 12,000      
Deferred leasing costs         3,270,000 3,521,000 3,840,000      
Difference between the estimated net basis and net assets of the rental property for federal income tax purposes 383,501,000       383,501,000          
Taxable income         30,208,000 $ 63,285,000 $ 73,546,008      
Deferred tax asset $ 16,000,000       16,000,000          
Income taxes, material adjustment amount         $ 0          
Common stock, shares outstanding 89,696,713     89,583,950 89,696,713 89,583,950     89,584,008  
Common units outstanding 10,488,105     10,516,844 10,488,105 10,516,844 11,083,876   10,516,844 11,864,775
LTIP units outstanding 657,373     657,373    
Distributions payable, record date Jan. 05, 2017     Jan. 06, 2016            
Distributions payable, approved date Dec. 13, 2016     Dec. 08, 2015            
Common stock dividends and common unit distributions per share $ 0.15     $ 0.15            
Dividends paid per common share         $ 0.60 $ 0.60 $ 0.90      
Dividends paid, percent representing ordinary income           90.00% 77.00%      
Dividends paid, percent representing return of capital to shareholders           10.00% 23.00%      
Dividends paid, percent representing capital gain         100.00%          
Restricted stock expense         $ 5,646,000 $ 2,219,000 $ 8,139,000      
Distributions payable, pay date Jan. 13, 2017     Jan. 15, 2016            
Executive Officers [Member]                    
Significant Accounting Policies [Line Items]                    
Restricted stock expense             $ 5,824,000      
Subsequent Event [Member]                    
Significant Accounting Policies [Line Items]                    
Common stock, shares outstanding               89,696,824    
Common units outstanding               10,488,105    
LTIP units outstanding               657,373