Leasing Activity (Narrative) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010
Tenant reimbursements	$ 14.7	$ 14.0	$ 30.2	$ 28.2
Percentage of leased asset subjected to termination options	5.90%		5.90%
Percentage of leased asset exercisable in period	2.20%		2.20%
Minimum [Member]
Non-cancelable leases term (in years)			1
Maximum [Member]
Non-cancelable leases term (in years)			10