Real Estate (Schedule Of Allocation Of The Aggregate Purchase Price Of Acquisition) (Details) (USD $) In Thousands, unless otherwise specified				0 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Oct. 04, 2012 Fountain Square [Member]	Aug. 29, 2012 680 Folsom Street [Member]	Mar. 01, 2012 453 Ravendale Drive [Member]	Mar. 13, 2012 100 Federal Street [Member]
Restructuring Cost and Reserve [Line Items]
Land				$ 56,853		$ 5,477	$ 131,067
Land Held For Future Development					3,058
Construction in Progress					117,046
Building and improvements				274,735		974	387,321
Tenant improvements				31,563		116	48,633
In-place lease intangibles				41,256	32,799	223	69,530
Above-market rents				6,240			81
Deferred Financing Fees					245
Other assets				35,774			4,800
Prepaid Expenses and Other Assets					7,473
Below-market rents				(7,089)		(140)	(22,515)
Above market assumed debt adjustment				(23,122)
Other Liabilities				(6,194)
Accounts payable and accrued expenses					(14,502)		(3,917)
Total aggregate consideration				410,016		6,700	615,000
Indebtedness assumed				(211,250)
Redeemable noncontrolling Interest	(98,787)	0	0	(98,787)
Net assets acquired				$ 99,979	$ 146,119	$ 6,650	$ 615,000