Debt of the Operating Partnership (Revolving Credit Facility) (Details) In Thousands, unless otherwise specified														6 Months Ended
	Jun. 30, 2011 USD ($)	Jun. 30, 2011 USD [Member] Revolving Credit Facility [Member] USD ($)	Dec. 31, 2010 USD [Member] Revolving Credit Facility [Member] USD ($)	Jun. 30, 2011 Euro [Member]	Dec. 31, 2010 Euro [Member]	Jun. 30, 2011 Euro [Member] Revolving Credit Facility [Member] EUR (€)		Jun. 30, 2011 British Sterling [Member]	Dec. 31, 2010 British Sterling [Member]	Jun. 30, 2011 British Sterling [Member] Revolving Credit Facility [Member] GBP (£)		Dec. 31, 2010 British Sterling [Member] Revolving Credit Facility [Member] GBP (£)		Jun. 30, 2011 Revolving Credit Facility [Member] USD ($)		Dec. 31, 2010 Revolving Credit Facility [Member] USD ($)
Maturity Date														Aug. 31, 2011	[1]
Balance/Principal Outstanding	$ 341,400	$ 251,000	$ 312,500			€ 18,655	[2]			£ 71,762	[3]	£ 21,034	[3]	$ 341,417	[4]	$ 333,534	[4]
US LIBOR, EURIBOR and GBP LIBOR maturity range minimum (months)														1
US LIBOR, EURIBOR and GBP LIBOR maturity range maximum (months)														6
US LIBOR, EURIBOR and GBP LIBOR margin range minimum														1.10%
US LIBOR, EURIBOR and GBP LIBOR margin range maximum														2.00%
Revolving credit facility rate calculation federal funds rate per annum														1/2 of 1%
Revolving credit facility rate calculation federal funds rate margin minimum														0.10%
Revolving credit facility rate calculation federal funds rate margin maximum														1.00%
Fee incurred for unused portion of credit facility minimum percentage														0.125%
Fee incurred for unused portion of credit facility maximum percentage														0.20%
Revolving credit facility commitments extension (years)														1
Weighted-average interest rate		1.39%	1.40%			2.50%				1.83%		1.69%		1.54%		1.42%
Exchange rate				1.45	1.34			1.61	1.56

[1] A one-year extension is available, which we may exercise if certain conditions are met.

[2] Based on exchange rate of $1.45 to €1.00 as of June 30, 2011.

[3] Based on exchange rate of $1.61 to £1.00 as of June 30, 2011 and $1.56 to £1.00 as of December 31, 2010.

[4] Balances as of June 30, 2011 and December 31, 2010 are as follows (balances, in thousands):