Real Estate Activity During The Six Months Ended June 30, 2012 (Schedule Of Allocation Of The Aggregate Purchase Price Of Acquisition) (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 13, 2012 100 Federal Street [Member]	Mar. 01, 2012 453 Ravendale Drive [Member]
Land	$ 131,067	$ 5,477
Building and improvements	387,321	974
Tenant improvements	48,633	116
In-place lease intangibles	69,530	223
Above-market rents	81
Other assets	4,800
Below-market rents	(22,515)	(140)
Accounts payable and accrued expenses	(3,917)
Net assets acquired	$ 615,000	$ 6,650