Significant Credit and Revenue Concentration - Narrative (Details)
3 Months Ended 9 Months Ended 12 Months Ended
Sep. 30, 2015
Customer
tenant
Sep. 30, 2014
Sep. 30, 2015
Customer
tenant
Dec. 31, 2014
Customer
tenant
Revenue, Major Customer [Line Items]        
Number of tenants 435   435 454
Revenues        
Revenue, Major Customer [Line Items]        
Number of tenants | Customer 0   0 0
Revenues | Minimum        
Revenue, Major Customer [Line Items]        
Rental revenue in percentage of total revenue     5.00% 5.00%
Shopko | Revenues        
Revenue, Major Customer [Line Items]        
Rental revenue in percentage of total revenue 9.90% 14.30%    
Shopko | Total assets | Customer Concentration Risk        
Revenue, Major Customer [Line Items]        
Rental revenue in percentage of total revenue     7.70% 10.70%
Shopko | Real estate investment portfolio | Customer Concentration Risk        
Revenue, Major Customer [Line Items]        
Rental revenue in percentage of total revenue     9.90% 13.10%