Annual report pursuant to Section 13 and 15(d)

Significant Accounting Policies (Narrative) (Details)

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Significant Accounting Policies (Narrative) (Details) - USD ($)
 3 Months Ended 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Jan. 03, 2018
Jan. 05, 2017
Dec. 31, 2014
Significant Accounting Policies [Line Items]                
Capitalized development and construction salaries and other related costs     $ 2,200,000 $ 2,600,000 $ 4,200,000      
Construction, tenant improvement, and development in-progress $ 559,500,000 $ 361,100,000 $ 559,500,000 361,100,000        
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     4,612,000 4,582,000 3,790,000      
Loss from extinguishment of debt, net     (421,000) 12,420,000      
Write off of unamortized deferred financing costs     400,000 700,000 0      
Deferred leasing costs     3,146,000 3,270,000 3,521,000      
Difference between the estimated net basis and net assets of the rental property for federal income tax purposes 231,067,000   231,067,000          
Taxable income     97,037,000 $ 30,208,000 $ 63,285,000      
Deferred tax asset $ 11,700,000   11,700,000          
Income taxes, material adjustment amount     $ 0          
Common stock, shares outstanding 89,914,113 89,696,713 89,914,113 89,696,713     89,696,824  
Common units outstanding 10,438,855 10,488,105 10,438,855 10,488,105 10,516,844   10,488,105 11,083,876
LTIP units outstanding 1,230,877 657,373 1,230,877 657,373   657,373
Distributions payable, record date Jan. 03, 2018 Jan. 05, 2017            
Distributions payable, approved date Dec. 12, 2017 Dec. 13, 2016            
Common stock dividends and common unit distributions per share $ 0.20 $ 0.15            
Dividends paid per common share     $ 0.70 $ 0.60 $ 0.60      
Dividends paid, percent representing ordinary income         90.00%      
Dividends paid, percent representing return of capital to shareholders       100.00% 10.00%      
Dividends paid, percent representing capital gain     100.00%          
Restricted stock expense     $ 7,447,000 $ 5,646,000 $ 2,219,000      
Distributions payable, pay date Jan. 12, 2018 Jan. 13, 2017            
Increase (decrease) to deferred tax assets     (5,300,000)          
Increase (decrease) to valuation allowance     $ (5,300,000)          
Subsequent Event [Member]                
Significant Accounting Policies [Line Items]                
Common stock, shares outstanding           89,914,658    
Common units outstanding           10,438,855    
LTIP units outstanding           1,230,877