Unsecured Lines of Credit Payable (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		3 Months Ended
	Jun. 30, 2013 Credit Facility No. 1 [Member]	Dec. 31, 2012 Credit Facility No. 1 [Member]	Jun. 30, 2013 Credit Facility No. 2 [Member]	Dec. 31, 2012 Credit Facility No. 2 [Member]	Jun. 30, 2013 Maximum [Member] Credit Facility No. 1 [Member]	Jun. 30, 2013 Maximum [Member] Credit Facility No. 2 [Member]	Jun. 30, 2013 5.125% unsecured notes [Member] Unsecured Debt [Member]	Mar. 15, 2013 5.125% unsecured notes [Member] Unsecured Debt [Member]
Line of Credit Facility [Line Items]
Expiration date	Jun. 30, 2015		Jul. 31, 2016
Committed capacity	$ 100,000		$ 400,000		$ 200,000	$ 600,000
Borrowings outstanding	(15,000)	0	(60,000)	0
Unused and available	85,000		340,000
Line of Credit Facility [Roll Forward]
Balance at December 31, 2012		0		0
Borrowings	30,000		60,000
Repayments	(15,000)		0
Balance at June 30, 2013	$ 15,000	$ 0	$ 60,000	$ 0
Interest rate, stated percentage							5.125%	5.125%