Debt (Details) - USD ($)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Debt
Total outstanding balances	$ 0	$ 18,000
Amount of interest costs incurred, capitalized, and expensed
Interest costs incurred	1,133,000	953,000	$ 1,019,000
Less: Interest capitalized	(136,000)	(98,000)	(118,000)
Interest expense	$ 997,000	855,000	$ 901,000
Base rate
Debt
Credit facility, interest rate basis	base rate
Line of credit
Debt
Credit facility, total available credit	$ 304,400,000
Primary Revolving Credit Facility
Debt
Credit facility, total available credit	300,000,000
Credit facility, remaining borrowing capacity	$ 4,400,000
Credit facility, interest rate low end of range (as a percent)	0.668%
Credit facility, interest rate high end of range (as a percent)	7.50%
Total outstanding balances	$ 0	$ 18,000
Primary Revolving Credit Facility | Minimum
Debt
Facility fees on the available commitment of the facility (as a percent)	0.15%
Primary Revolving Credit Facility | Maximum
Debt
Facility fees on the available commitment of the facility (as a percent)	0.30%
Primary Revolving Credit Facility | LIBOR
Debt
LIBOR Rate	0.36%
Credit facility, interest rate basis	LIBOR
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%