Debt - Narrative - Revolving Credit Facilities (Details) (USD $)
 0 Months Ended 12 Months Ended
May 20, 2014
Jul. 17, 2013
Jul. 17, 2014
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Line of Credit Facility [Line Items]            
Extended term of debt 17 years          
Amount outstanding       $ 15,181,000 $ 35,120,000  
Line of Credit [Member]            
Line of Credit Facility [Line Items]            
Line of credit facility maximum borrowing capacity   400,000,000.0        
Extended term of debt   12 months        
Line of Credit [Member] | Minimum [Member]            
Line of Credit Facility [Line Items]            
Commitment fee on unused capacity   0.25%        
Line of Credit [Member] | Maximum [Member]            
Line of Credit Facility [Line Items]            
Commitment fee on unused capacity   0.35%        
Line of Credit [Member] | Base Rate [Member] | Minimum [Member]            
Line of Credit Facility [Line Items]            
Basis spread on variable rate   1.00%        
Line of Credit [Member] | Base Rate [Member] | Maximum [Member]            
Line of Credit Facility [Line Items]            
Basis spread on variable rate   2.00%        
Line of Credit [Member] | LIBOR [Member] | Minimum [Member]            
Line of Credit Facility [Line Items]            
Basis spread on variable rate   2.00%        
Line of Credit [Member] | LIBOR [Member] | Maximum [Member]            
Line of Credit Facility [Line Items]            
Basis spread on variable rate   3.00%        
Revolving Credit Facility            
Line of Credit Facility [Line Items]            
Maturity term     3 years      
Line of credit facility maximum borrowing capacity   100,000,000.0        
Commitment fee       1,200,000 700,000 100,000
Deferred Costs   4,500,000        
Interest rate excluding noncash amortization of deferred financing costs and non-utilization fee       2.69%    
Amount outstanding       0    
Line of credit facility remaining borrowing capacity       400,000,000    
Revolving Credit Facility | Special Purpose Entity [Member]            
Line of Credit Facility [Line Items]            
Maturity term       24 months    
Amount outstanding       15,200,000    
Maximum borrowing capacity increase       40,000,000.0    
Number of separate advances       3    
Weighted average interest rate       5.83%    
Revolving Credit Facility | Deferred Charges and Other Assets [Member]            
Line of Credit Facility [Line Items]            
Deferred Costs       $ 2,300,000 $ 3,800,000  
Revolving Credit Facility | Weighted Average [Member] | Special Purpose Entity [Member]            
Line of Credit Facility [Line Items]            
Weighted average stated interest rate       3.58%    
Revolving Credit Facility | Floor Rate [Member] | Special Purpose Entity [Member]            
Line of Credit Facility [Line Items]            
Basis spread on variable rate       3.50%