Debt - Revolving Credit Facilities - Narrative (Details) - USD ($)
 3 Months Ended 6 Months Ended 12 Months Ended
Mar. 31, 2015
Jun. 30, 2015
Jun. 30, 2015
Dec. 31, 2013
Dec. 31, 2014
Line of Credit Facility [Line Items]          
Amount outstanding   $ 20,000,000 $ 20,000,000   $ 15,114,000
Weighted Average          
Line of Credit Facility [Line Items]          
Weighted Average Term     5 years 3 months 18 days    
Unsecured Debt          
Line of Credit Facility [Line Items]          
Line of credit facility maximum borrowing capacity $ 600,000,000.0        
Line of credit facility, accordion feature, increase limit 1,000,000,000.0        
Commitment fee on unused capacity   0.25%      
Origination costs $ 3,700,000        
Unamortized deferred financing costs   $ 3,500,000 $ 3,500,000    
Amount outstanding   20,000,000 20,000,000    
Letters of credit issued   18,000,000 18,000,000    
Line of credit facility remaining borrowing capacity   $ 562,000,000 $ 562,000,000    
Unsecured Debt | Minimum          
Line of Credit Facility [Line Items]          
Commitment fee on unused capacity 0.125%        
Unsecured Debt | Maximum          
Line of Credit Facility [Line Items]          
Commitment fee on unused capacity 0.30%        
Unsecured Debt | Weighted Average          
Line of Credit Facility [Line Items]          
Weighted Average Term     6 years 6 months 18 days    
Unsecured Debt | LIBOR          
Line of Credit Facility [Line Items]          
Basis spread on variable rate   1.70%      
Unsecured Debt | LIBOR | Minimum          
Line of Credit Facility [Line Items]          
Basis spread on variable rate 0.875%        
Unsecured Debt | LIBOR | Maximum          
Line of Credit Facility [Line Items]          
Basis spread on variable rate 1.55%        
Unsecured Debt | Swingline Loans          
Line of Credit Facility [Line Items]          
Line of credit facility maximum borrowing capacity $ 50,000,000.0        
Unsecured Debt | Letters of Credit          
Line of Credit Facility [Line Items]          
Line of credit facility maximum borrowing capacity 60,000,000.0        
Line of Credit          
Line of Credit Facility [Line Items]          
Commitment fee on unused capacity       0.35%  
Debt extinguishment costs $ 2,000,000        
Line of Credit | LIBOR          
Line of Credit Facility [Line Items]          
Basis spread on variable rate       2.50%  
Revolving Credit Facility          
Line of Credit Facility [Line Items]          
Line of credit facility maximum borrowing capacity       $ 400,000,000.0  
Revolving Credit Facility | Special Purpose Entity          
Line of Credit Facility [Line Items]          
Line of credit facility maximum borrowing capacity   $ 40,000,000.0 $ 40,000,000.0    
Line of credit facility remaining borrowing capacity   $ 40,000,000 $ 40,000,000    
Weighted Average Term     24 months