Indebtedness (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended	9 Months Ended
	Sep. 30, 2013	Sep. 30, 2013	Dec. 31, 2012
Indebtedness
Unsecured revolving credit facility	$ 69,000	$ 69,000	$ 49,500
Mortgage notes payable	91,343	91,343	93,127
Total	510,343	510,343	492,627
Number of assumed secured mortgage loans		4
Aggregate net book value of secured properties	120,798	120,798
Number of properties secured by mortgage notes	5	5
Unsecured revolving credit facility due in 2015
Indebtedness
Unsecured revolving credit facility	69,000	69,000	49,500
Maximum borrowing capacity on revolving credit facility	550,000	550,000
Revolving credit facility, interest rate basis		LIBOR
Maximum borrowing capacity on debt instruments may be increased under certain conditions	1,100,000	1,100,000
Option to extend the maturity date subject to certain conditions and the payment of a fee		1 year
Interest rate premium (as a percent)		1.50%
Facility fee (as a percent)		0.35%
Interest rate (as a percent)	1.70%	1.70%
The weighted average annual interest rate (as a percent)	1.70%	1.70%
Amount available to be drawn	481,000	481,000
Unsecured term loan, due in 2017
Indebtedness
Term loan	350,000	350,000	350,000
Maximum borrowing capacity on debt instruments may be increased under certain conditions	700,000	700,000
Term loan, interest rate basis		LIBOR
Interest rate premium (as a percent)		1.75%
Interest rate (as a percent)	1.90%	1.90%
The weighted average annual interest rate (as a percent)	1.90%	1.90%
5.73% Mortgage notes due in 2015
Indebtedness
Mortgage notes payable	48,607	48,607	49,274
Interest rate (as a percent)	5.73%	5.73%	5.73%
Unamortized fair value premium included in mortgage notes	463	463
6.21% Mortgage notes due in 2016
Indebtedness
Mortgage notes payable	24,223	24,223	24,441
Interest rate (as a percent)	6.21%	6.21%	6.21%
7% Mortgage notes due in 2019
Indebtedness
Mortgage notes payable	10,003	10,003	10,247
Interest rate (as a percent)	7.00%	7.00%	7.00%
Unamortized fair value premium included in mortgage notes	782	782
8.15% Mortgage notes due in 2021
Indebtedness
Mortgage notes payable	8,510	8,510	9,165
Interest rate (as a percent)	8.15%	8.15%	8.15%
Unamortized fair value premium included in mortgage notes	$ 556	$ 556