Indebtedness (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended		12 Months Ended
	Dec. 31, 2013 loan item property	Dec. 31, 2012	Dec. 31, 2013 Unsecured revolving credit facility due in 2015	Dec. 31, 2012 Unsecured revolving credit facility due in 2015	Dec. 31, 2013 Unsecured term loan, due in 2017	Dec. 31, 2012 Unsecured term loan, due in 2017	Dec. 31, 2013 5.73% Mortgage notes due in 2015	Dec. 31, 2012 5.73% Mortgage notes due in 2015	Dec. 31, 2013 6.21% Mortgage notes due in 2016	Dec. 31, 2012 6.21% Mortgage notes due in 2016	Dec. 31, 2013 7% Mortgage notes due in 2019	Dec. 31, 2012 7% Mortgage notes due in 2019	Dec. 31, 2013 8.15% Mortgage notes due in 2021	Dec. 31, 2012 8.15% Mortgage notes due in 2021
Indebtedness
Unsecured revolving credit facility	$ 157,000	$ 49,500	$ 157,000	$ 49,500
Term loan					350,000	350,000
Mortgage notes payable	90,727	93,127					48,377	49,274	24,147	24,441	9,919	10,247	8,284	9,165
Total	597,727	492,627
Interest rate (as a percent)							5.73%	5.73%	6.21%	6.21%	7.00%	7.00%	8.15%	8.15%
Unamortized fair value premium included in mortgage notes							409	621			749	878	525	651
Maximum borrowing capacity on revolving credit facility			550,000
Option to extend the maturity date subject to certain conditions and the payment of a fee			1 year
Maximum borrowing capacity on debt instruments may be increased under certain conditions			1,100,000		700,000
Revolving credit facility, interest rate basis			LIBOR
Term loan, interest rate basis					LIBOR
Interest rate premium (as a percent)			1.50%		1.75%
Facility fee (as a percent)			0.35%
Interest rate (as a percent)			1.67%		1.91%
The weighted average annual interest rate (as a percent)			1.68%	1.75%	1.94%	1.99%
Amount available to be drawn			393,000
Number of assumed secured mortgage loans	4
Aggregate net book value of secured properties	120,595
Number of properties secured by mortgage notes	4
Number of buildings secured by mortgage notes	5
Repayment of debt
2014	2,072
2015	205,691
2016	24,708
2017	351,307
2018	1,415
Thereafter	10,851
Aggregate outstanding debt	$ 596,044