Revolving Credit Facility (Details) (USD $)	1 Months Ended	3 Months Ended
	Nov. 30, 2011	Mar. 31, 2012	Mar. 31, 2011	Dec. 31, 2011
Long-term Line of Credit		$ 95,300,000		$ 97,300,000
Borrowings on term loan		0	50,000,000
Unused balance of the unsecured revolving credit facility		104,700,000
Interest Expense		499,000	477,000
Amortization of loan costs		76,000	36,000
Debt Instrument, Maturity Date Range, End	4
Gain (Loss) on Interest Rate Derivative Instruments Not Designated as Hedging Instruments			(1,254,000)
Wells Fargo et al. [Member]
Line of Credit Facility, Maximum Borrowing Capacity	200,000,000
Number of basis points over LIBOR	0.015
Revolving credit facility interest rate		1.75%
Increase in total credit facility	300,000,000
Line of Credit Facility, Unused Capacity, Commitment Fee Basis Points	0.0035
Debt Instrument, Maturity Date Range, End, Extension	5