Organization - Narrative (Details)
$ in Thousands
6 Months Ended
Jun. 30, 2015
USD ($)
Property
Dec. 31, 2014
USD ($)
Property
Summary Of Business And Significant Accounting Policies [Line Items]    
Gross investment in real estate properties | $ $ 8,207,792 $ 8,044,363
Number of real estate properties 2,600 2,509
Owned Properties    
Summary Of Business And Significant Accounting Policies [Line Items]    
Gross investment in real estate properties | $ $ 8,098,415 $ 7,934,938
Number of real estate properties 2,455 2,364
Gross investment in real estate, percentage 98.70%  
Number of owned real estate properties 2,455  
Financed Properties    
Summary Of Business And Significant Accounting Policies [Line Items]    
Gross investment in real estate properties | $ $ 109,377 $ 109,425
Number of real estate properties 145 145
Securing mortgage properties, percentage 1.30%  
Securing mortgage properties 145  
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%