Debt of the Operating Partnership (Revolving Credit Facility) (Narrative) (Details) (USD $)	3 Months Ended			6 Months Ended
	Jun. 30, 2011		Jun. 30, 2010	Jun. 30, 2011		Jun. 30, 2010	Dec. 31, 2010
Revolving credit facility maturity date				August 31, 2011
Balance/Principal Outstanding	$ 341,400,000			$ 341,400,000
Revolving Credit Facility [Member]
Revolving credit facility borrowing capacity	750,000,000			750,000,000
Line of credit facility, revolving credit, description				The bank group is obligated to grant the extension option provided we give proper notice, we make certain representations and warranties and no default exists under the revolving credit facility
Revolving credit facility commitments extension (years)				1
Line of credit facility interest rate description				As of June 30, 2011, borrowings under the revolving credit facility bore interest at a blended rate of 1.39% (U.S), 2.50% (Euro) and 1.83% (GBP), which are based on 1-month LIBOR, 1-month EURIBOR and 1-month GBP LIBOR, respectively, plus a margin of 1.20%
Sub-facility borrowing capacity	515,000,000			515,000,000
Balance/Principal Outstanding	341,417,000	[1]		341,417,000	[1]		333,534,000	[1]
Letter of credit security amount	30,500,000			30,500,000
Maximum distribution of funds from operations, percentage				95.00%
Interest costs, capitalized during period	$ 4,200,000		$ 2,500,000	$ 8,900,000		$ 4,400,000

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