Significant Credit and Revenue Concentration - Narrative (Details)
3 Months Ended 12 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Dec. 31, 2014
Revenue, Major Customer [Line Items]      
Number of tenants 474   454
Shopko      
Revenue, Major Customer [Line Items]      
Rental revenue in percentage of total revenue 12.80% 14.00%  
Rental revenue by combined properties operated by tenants 12.30%   13.10%
Revenues      
Revenue, Major Customer [Line Items]      
Number of tenants 0 0  
Revenues | Minimum      
Revenue, Major Customer [Line Items]      
Rental revenue in percentage of total revenue 5.00% 5.00%