Debt (Details) (USD $)	12 Months Ended					1 Months Ended	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2011 Primary revolving line of credit	Dec. 31, 2011 Other revolving credit lines	Oct. 31, 2007 Unsecured credit agreement Y	Dec. 31, 2011 Unsecured credit agreement Y	Dec. 31, 2011 Unsecured credit agreement Minimum	Dec. 31, 2011 Unsecured credit agreement Less than	Dec. 31, 2011 Unsecured credit agreement LIBOR	Dec. 31, 2011 Unsecured credit agreement Base rate
Revolving line of credit
Credit facility, maximum borrowing capacity				$ 200,000,000	$ 4,400,000	$ 400,000,000	$ 400,000,000
Credit facility, interest rate basis				LIBOR						LIBOR	Base rate
Credit facility, interest rate spread (as a percent)				0.27%							0.50%
Credit facility, interest rate at end of period (as a percent)				0.55%
Commitment fees payable on unused portion of the facility (as a percent)				0.08%
Credit facility, interest rate low end of range (as a percent)					2.20%					0.27%
Credit facility, interest rate high end of range (as a percent)					3.50%					0.40%
Term of revolving credit facility (in years)						5	5
Credit facility, current borrowing capacity						200,000,000	200,000,000
Credit facility, available increase in borrowing capacity						200,000,000	200,000,000
Credit facility, annual facility fee (as a percent)								0.08%	0.10%
Amount of interest costs incurred, capitalized, and expensed
Interest costs incurred	661,000	518,000	471,000
Less: Interest capitalized	(88,000)	(174,000)	(104,000)
Interest expense	$ 573,000	$ 344,000	$ 367,000