Real Estate Activity During The Three Months Ended March 31, 2012 (Schedule Of Allocation Of The Aggregate Purchase Price Of Acquisition) (Details) (USD $) In Thousands, unless otherwise specified	Mar. 01, 2012 453 Ravendale Drive [Member]	Mar. 13, 2012 100 Federal Street [Member]
Restructuring Cost and Reserve [Line Items]
Land	$ 5,477	$ 131,067
Building and improvements	974	387,321
Tenant improvements	116	48,633
In-place lease intangibles	223	69,530
Above market rents		81
Other assets		4,800
Below market rents	(140)	(22,515)
Accounts payable and accrued expenses		(3,917)
Net assets acquired	$ 6,650	$ 615,000