Real Estate Activity During Nine Months Ended September 30, 2012 (Schedule Of Allocation Of 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]	Aug. 29, 2012 680 Folsom Street [Member]
Restructuring Cost and Reserve [Line Items]
Land	$ 131,067	$ 5,477
Construction in progress			117,046
Building and improvements	387,321	974
Land held for future development			3,058
Tenant improvements	48,633	116
In-place lease intangibles	69,530	223	32,799
Above-market rents	81
Deferred financing costs			245
Other assets	4,800
Prepaid expenses and other assets			7,473
Below-market rents	(22,515)	(140)
Accounts payable and accrued expenses	(3,917)		(14,502)
Net assets acquired	$ 615,000	$ 6,650	$ 146,119