Debt (Details) (USD $)	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Debt
Credit facility, maximum borrowing capacity	$ 304,400,000
Amount of interest costs incurred, capitalized, and expensed
Interest costs incurred	1,019,000	909,000	661,000
Less: Interest capitalized	(118,000)	(116,000)	(88,000)
Interest expense	901,000	793,000	573,000
Primary revolving line of credit
Debt
Credit facility, maximum borrowing capacity	300,000,000
Primary revolving line of credit | Maximum
Debt
Credit facility, interest rate spread (as a percent)	0.45%
Facility fees on the available commitment of the facility (as a percent)	0.30%
Primary revolving line of credit | Minimum
Debt
Credit facility, interest rate spread (as a percent)	0.00%
Facility fees on the available commitment of the facility (as a percent)	0.15%
Primary revolving line of credit | LIBOR
Debt
Credit facility, interest rate basis	LIBOR
LIBOR Rate at end of period (as a percent)	0.77%
Primary revolving line of credit | LIBOR | Maximum
Debt
Credit facility, interest rate spread (as a percent)	1.45%
Primary revolving line of credit | LIBOR | Minimum
Debt
Credit facility, interest rate spread (as a percent)	0.60%
Primary revolving line of credit | Base rate
Debt
Credit facility, interest rate basis	Base rate
Other revolving credit lines and long term debt
Debt
Total Borrowing Capacity	4,500,000
Credit facility, interest rate low end of range (as a percent)	1.084%
Credit facility, interest rate high end of range (as a percent)	7.25%
Total outstanding balances	$ 100,000	$ 200,000