Use of Derivative Financial Instruments (Details) (USD $)	3 Months Ended	12 Months Ended	0 Months Ended	3 Months Ended			3 Months Ended								0 Months Ended	3 Months Ended
	Mar. 31, 2013	Dec. 31, 2012	Feb. 14, 2013 Wells Fargo Bank, N.A. Unsecured Term Loan, due on February 14, 2020	Mar. 31, 2013 Wells Fargo Bank, N.A. Unsecured Term Loan, due on February 14, 2020	Mar. 31, 2013 Minimum Wells Fargo Bank, N.A. Unsecured Term Loan, due on February 14, 2020	Mar. 31, 2013 Maximum Wells Fargo Bank, N.A. Unsecured Term Loan, due on February 14, 2020	Mar. 31, 2013 Interest rate swap one	Mar. 31, 2013 Interest rate swap two	Mar. 31, 2013 Interest rate swap three	Mar. 31, 2013 Interest rate swap four	Mar. 31, 2013 Interest rate swap five	Mar. 31, 2013 Interest rate swap six	Mar. 31, 2013 Interest rate swap seven	Mar. 31, 2013 Interest rate swap eight	Mar. 01, 2013 Interest rate swap eight Wells Fargo Bank, N.A. Unsecured Term Loan, due on February 14, 2020	Mar. 31, 2013 Interest rate swap eight Wells Fargo Bank, N.A. Unsecured Term Loan, due on February 14, 2020	Mar. 01, 2013 Interest rate swap eight Minimum Wells Fargo Bank, N.A. Unsecured Term Loan, due on February 14, 2020	Mar. 01, 2013 Interest rate swap eight Maximum Wells Fargo Bank, N.A. Unsecured Term Loan, due on February 14, 2020
Risk Management Objective of Using Derivatives
Portion of Unsecured Term Loan swapped from LIBOR for fixed rate				$ 25,000,000											$ 25,000,000
Variable rate received under agreement							One-month LIBOR	One-month LIBOR	One-month LIBOR	One-month LIBOR	One-month LIBOR	One-month LIBOR	One-month LIBOR	One-month LIBOR		One-month LIBOR
Variable Rate of Interest	One-month LIBOR	One-month LIBOR	One-month LIBOR	LIBOR
Unsecured Term Loan			150,000,000	150,000,000											150,000,000
Interest rate margin (as a percent)			2.15%	2.15%	2.15%	2.70%											2.15%	2.70%
Derivative, Fixed Interest Rate				1.33%			0.7945%	0.7945%	0.7945%	0.7945%	0.7975%	0.7525%	0.727%	1.33%	1.33%
Spread over fixed rate (as a percent)																	2.15%	2.70%
Unsecured Term Loan Swaps
Notional Amount							$ 10,000,000	$ 10,000,000	$ 10,000,000	$ 10,000,000	$ 10,000,000	$ 25,000,000	$ 25,000,000	$ 25,000,000	$ 25,000,000
Fixed Interest Rate (as a percent)				1.33%			0.7945%	0.7945%	0.7945%	0.7945%	0.7975%	0.7525%	0.727%	1.33%	1.33%
Variable Interest Rate							One-month LIBOR	One-month LIBOR	One-month LIBOR	One-month LIBOR	One-month LIBOR	One-month LIBOR	One-month LIBOR	One-month LIBOR		One-month LIBOR