Indebtedness (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended				3 Months Ended					3 Months Ended
	Mar. 31, 2014	Dec. 31, 2013	Mar. 31, 2014 Real estate properties held for sale property	Dec. 31, 2013 Real estate properties held for sale	Mar. 31, 2014 Unsecured revolving credit facility, due October 2015	Mar. 31, 2013 Unsecured revolving credit facility, due October 2015	Mar. 31, 2014 Unsecured term loan, due December 2016	Mar. 31, 2013 Unsecured term loan, due December 2016	Mar. 31, 2014 Mortgage notes maturing from 2014 through 2026 property	Mar. 31, 2014 Mortgage Notes Payable, due 2014 at 4.95%
Indebtedness
Prepayment of debt										$ 11,988
Outstanding indebtedness							500,000
Maximum borrowing capacity					750,000
Number of calendar days in forbearance period following the replacement of Board of Trustees	21 days
Minimum percentage of maintenance of net cash proceeds received from asset dispositions	80.00%
Variable rate basis					LIBOR		LIBOR
Basis points on variable rate (as a percent)					1.50%		1.85%
Facility fee of lending commitments under our revolving credit facility (as a percent)					0.35%
Interest rate payable (as a percent)					1.70%		2.00%
Average interest rate (as a percent)					1.70%	1.70%	2.00%	2.10%
Addition in prevailing interest rate (as a percent)					4.00%		4.00%
Amount outstanding					235,000
Number of Real Estate Properties Secured by Mortgage			2						12
Number of buildings secured by mortgage			3						17
Cost of properties mortgaged	5,915,516	5,537,165	201,557	536,552					1,250,900
Mortgaged properties aggregate net book value	4,980,740	4,642,106							1,087,104
Mortgage notes payable, net	$ 898,804	$ 914,510	$ 19,688	$ 20,018					$ 898,804