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Debt (Details) - USD ($)
 12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Amount of interest costs incurred, capitalized, and expensed      
Interest costs incurred $ 1,167,000 $ 1,133,000 $ 953,000
Less: Interest capitalized (20,000) (136,000) (98,000)
Interest expense $ 1,147,000 $ 997,000 $ 855,000
Base rate      
Debt      
Credit facility, interest rate basis base rate    
Line of credit      
Debt      
Credit facility, total available credit $ 303,800,000    
Primary Revolving Credit Facility      
Debt      
Credit facility, total available credit 300,000,000    
Credit facility, remaining borrowing capacity 3,800,000    
Total outstanding balances $ 0    
Primary Revolving Credit Facility | Minimum      
Debt      
Facility fees on the available commitment of the facility (as a percent) 0.15%    
Debt Instrument, Interest Rate, Stated Percentage 0.481%    
Primary Revolving Credit Facility | Maximum      
Debt      
Facility fees on the available commitment of the facility (as a percent) 0.30%    
Debt Instrument, Interest Rate, Stated Percentage 7.75%    
Primary Revolving Credit Facility | LIBOR      
Debt      
LIBOR Rate 0.72%    
Primary Revolving Credit Facility | LIBOR | Minimum      
Debt      
Credit facility, interest rate spread (as a percent) 0.60%    
Primary Revolving Credit Facility | LIBOR | Maximum      
Debt      
Credit facility, interest rate spread (as a percent) 1.45%    
Primary Revolving Credit Facility | Base rate | Minimum      
Debt      
Credit facility, interest rate spread (as a percent) 0.00%    
Primary Revolving Credit Facility | Base rate | Maximum      
Debt      
Credit facility, interest rate spread (as a percent) 0.45%