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, 2013
Dec. 31, 2012
Dec. 31, 2013
property
Dec. 31, 2012
Dec. 31, 2011
Jan. 06, 2014
Jan. 04, 2013
Dec. 31, 2010
Significant Accounting Policies [Abstract]                
Capitalized development and construction salaries and other related costs     $ 4,400,000 $ 3,800,000 $ 3,700,000      
Construction, tenant improvement, and development in-progress 40,800,000 107,600,000 40,800,000 107,600,000        
Number of properties placed into service     2          
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     3,172,000 2,669,000 2,370,000      
Write off of unamortized deferred financing costs     156,000 593,000 0      
Deferred leasing costs     4,223,000 4,354,000 4,432,000      
Deferred tax asset 13,100,000   13,100,000          
Income taxes, material adjustment amount     0          
Common stock, shares outstanding 88,247,591 87,536,292 88,247,591 87,536,292   87,928,002 87,537,250  
Common units outstanding 11,864,775 12,141,836 11,864,775 12,141,836 12,197,122 11,864,775 12,141,836 13,007,668
Distributions payable, record date Jan. 06, 2014 Jan. 04, 2013            
Common stock dividends and common unit distributions per share $ 0.30 $ 0.45            
Dividends paid per common share     $ 1.50 $ 1.80 $ 1.80      
Dividends paid, percent representing ordinary income     53.00% 75.00% 77.00%      
Dividends paid, percent representing return of capital to shareholders     33.00% 25.00% 23.00%      
Dividends paid, percent representing capital gain     14.00%          
Stock compensation     2,387,000 3,642,000 4,123,000      
Common stock dividends and common unit distributions payable, date Jan. 15, 2014 Jan. 11, 2013            
Difference between other comprehensive income and net income     0 0 0      
Accumulated other comprehensive income $ 0 $ 0 $ 0 $ 0