Leasing Activities (Schedule of Lease Receivables) (Details) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Operating Lease Receivables [Line Items]
Tenant Reimbursements	$ 174,200,000	$ 190,800,000	$ 190,000,000
Operating Leases, Future Minimum Payments Receivable, Current	667,886,000
Operating Leases, Future Minimum Payments Receivable, in Two Years	628,433,000
Operating Leases, Future Minimum Payments Receivable, in Three Years	564,516,000
Operating Leases, Future Minimum Payments Receivable, in Four Years	499,342,000
Operating Leases, Future Minimum Payments Receivable, in Five Years	426,569,000
Operating Leases, Future Minimum Payments Receivable, Thereafter	1,630,698,000
Operating Leases, Future Minimum Payments Receivable	$ 4,417,444,000