Organization - Narrative (Details) (USD $)
3 Months Ended 0 Months Ended
Mar. 31, 2015
Property
Mar. 31, 2015
Dec. 31, 2014
Property
Summary Of Business And Significant Accounting Policies [Line Items]      
Gross investment in real estate properties $ 8,226,205,000 8,226,205,000 $ 8,044,363,000
Number of real estate properties 2,547 2,547 2,509
Number of shares sold (in shares) 418,935,311 418,935,311 411,824,039
Revenues 200,000    
Number of properties sold 15    
Gross sales proceeds from sale of real estate 77,200,000    
Current Year Acquisitions      
Summary Of Business And Significant Accounting Policies [Line Items]      
Gross investment in real estate properties 265,500,000 265,500,000  
Number of properties acquired 53    
Unsecured Debt      
Summary Of Business And Significant Accounting Policies [Line Items]      
Line of credit facility maximum borrowing capacity 600,000,000.0 600,000,000.0  
Maximum borrowing capacity increase, accordion feature 1,000,000,000.0 1,000,000,000.0  
Revolving Credit Facility      
Summary Of Business And Significant Accounting Policies [Line Items]      
Line of credit facility maximum borrowing capacity 400,000,000.0 400,000,000.0  
ATM Program      
Summary Of Business And Significant Accounting Policies [Line Items]      
Number of shares sold (in shares) 6,600,000 6,600,000  
Aggregate gross proceeds from issuance of common stock 79,800,000    
Aggregate net sales proceeds from Issuance of common stock 78,600,000    
General Partner      
Summary Of Business And Significant Accounting Policies [Line Items]      
Percentage ownership of operating partnership 1.00%    
Limited Partner      
Summary Of Business And Significant Accounting Policies [Line Items]      
Percentage ownership of operating partnership 99.00%    
LIBOR | Unsecured Debt      
Summary Of Business And Significant Accounting Policies [Line Items]      
Basis spread on variable rate   1.70%  
Owned Properties      
Summary Of Business And Significant Accounting Policies [Line Items]      
Gross investment in real estate properties 8,118,802,000 8,118,802,000 7,934,938,000
Number of real estate properties 2,402 2,402 2,364
Gross investment in real estate, percentage 98.70%    
Number of owned real estate properties 2,402 2,402  
Financed Properties      
Summary Of Business And Significant Accounting Policies [Line Items]      
Gross investment in real estate properties $ 107,403,000 107,403,000 $ 109,425,000
Number of real estate properties 145 145 145
Securing mortgage properties, percentage 1.30%    
Securing mortgage properties 145 145