Leasing Activities (Schedule of Lease Receivables) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Operating Lease Receivables [Line Items]
Tenant Reimbursements	$ 190,800	$ 190,000	$ 191,000
Operating Leases, Future Minimum Payments Receivable, Current	605,615
Operating Leases, Future Minimum Payments Receivable, in Two Years	568,550
Operating Leases, Future Minimum Payments Receivable, in Three Years	490,939
Operating Leases, Future Minimum Payments Receivable, in Four Years	421,655
Operating Leases, Future Minimum Payments Receivable, in Five Years	353,900
Operating Leases, Future Minimum Payments Receivable, Thereafter	1,116,410
Operating Leases, Future Minimum Payments Receivable	$ 3,557,069