Unsecured Lines Of Credit Payable - Narrative (Details) (USD $)	12 Months Ended			12 Months Ended			12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2012 Credit Facility No. 1 [Member]	Dec. 31, 2012 Credit Facility No. 1 [Member] London Interbank Offered Rate (LIBOR) [Member]	Dec. 31, 2012 Credit Facility No. 1 [Member] Maximum [Member]	Dec. 31, 2012 Credit Facility No. 2 [Member]	Dec. 31, 2012 Credit Facility No. 2 [Member] London Interbank Offered Rate (LIBOR) [Member]	Dec. 31, 2012 Credit Facility No. 2 [Member] Maximum [Member]	Dec. 31, 2012 Unsecured Debt [Member] 10 Year Unsecured Notes 5.05 % [Member]	Apr. 30, 2012 Unsecured Debt [Member] 10 Year Unsecured Notes 5.05 % [Member]	Dec. 31, 2012 Unsecured Debt [Member] 10 Year Unsecured Notes 3.950 % [Member]	Sep. 30, 2010 Unsecured Debt [Member] 10 Year Unsecured Notes 3.950 % [Member]
Line of Credit Facility [Line Items]
Committed capacity	$ 500,000,000	$ 475,000,000	$ 337,000,000	$ 100,000,000		$ 200,000,000	$ 400,000,000		$ 600,000,000
Stated interest rate		3.875%								5.05%	5.05%	3.95%	3.95%
Expiration date				Jun. 30, 2015			Jul. 31, 2016
Spread on variable rate					1.20%			1.20%
Extension term	1 year
Facility fee	0.25%