Indebtedness (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2012 loan property	Dec. 31, 2011
Indebtedness
Unsecured revolving credit facility		$ 345,500
Mortgage notes payable	94,826	95,383
Total	444,826	440,883
Number of assumed secured mortgage loans	4
Aggregate net book value of secured properties	123,825
Properties	5
Unsecured revolving credit facility due in 2015
Indebtedness
Unsecured revolving credit facility		345,500
Maximum borrowing capacity on revolving credit facility	550,000
Revolving credit facility, interest rate basis	LIBOR
Maximum borrowing capacity on credit facility may be increased under certain conditions	1,100,000
The weighted average annual interest rate for revolving credit facilities (as a percent)	1.80%
5.73% Mortgage notes due in 2015
Indebtedness
Mortgage notes payable	49,907	50,118
Interest rate (as a percent)	5.73%	5.73%
Unamortized fair value premium included in mortgage notes	775	775
6.21% Mortgage notes due in 2016
Indebtedness
Mortgage notes payable	24,645	24,713
Interest rate (as a percent)	6.21%	6.21%
7% Mortgage notes due in 2019
Indebtedness
Mortgage notes payable	10,483	10,559
Interest rate (as a percent)	7.00%	7.00%
Unamortized fair value premium included in mortgage notes	974	974
8.15% Mortgage notes due in 2021
Indebtedness
Mortgage notes payable	9,791	9,993
Interest rate (as a percent)	8.15%	8.15%
Unamortized fair value premium included in mortgage notes	749	749
Unsecured term loan
Indebtedness
Term loan	350,000
Maximum borrowing capacity on credit facility may be increased under certain conditions	$ 700,000
Term loan, interest rate basis	LIBOR
The weighted average annual interest rate for revolving credit facilities (as a percent)	2.02%
Term of loan (in years)	5