Segment Disclosure (Details 1) (USD $) In Thousands	Sep. 30, 2011		Dec. 31, 2010		Sep. 30, 2010	Dec. 31, 2009
Assets:
Land, buildings and improvement, net	$ 2,687,315		$ 2,254,077
Undeveloped land and construction in progress	328,785		290,365
Total Assets	3,367,684		2,816,565
Reconciliation to Consolidated Assets:
Total assets allocated to segments	3,367,684		2,816,565
Other unallocated asset:
Cash and cash equivalents	15,481		14,840		8,313	9,883
Restricted cash	25,436		1,461
Marketable securities	5,213		4,902
Deferred financing costs, net	19,638		16,447
Prepaid expenses and other assets, net	19,531		8,097
Total Assets	3,367,684		2,816,565
Operating Segments [Member]
Assets:
Total Assets	3,282,385	[1]	2,770,818	[1]
Reconciliation to Consolidated Assets:
Total assets allocated to segments	3,282,385	[1]	2,770,818	[1]
Other unallocated asset:
Total Assets	3,282,385	[1]	2,770,818	[1]
Reportable Segment - Office Properties [Member]
Assets:
Land, buildings and improvement, net	2,541,029		2,108,019
Undeveloped land and construction in progress	328,785		290,365
Total Assets	3,121,084	[1]	2,611,206	[1]
Reconciliation to Consolidated Assets:
Total assets allocated to segments	3,121,084	[1]	2,611,206	[1]
Other unallocated asset:
Total Assets	3,121,084	[1]	2,611,206	[1]
Non-Reportable Segment - Industrial Properties [Member]
Assets:
Land, buildings and improvement, net	146,286		146,058
Total Assets	161,301	[1]	159,612	[1]
Reconciliation to Consolidated Assets:
Total assets allocated to segments	161,301	[1]	159,612	[1]
Other unallocated asset:
Total Assets	$ 161,301	[1]	$ 159,612	[1]

[1] Includes land, buildings, and improvements, undeveloped land and construction in progress, current receivables, deferred rent receivables, deferred leasing costs, and acquisition-related intangible assets, all shown on a net basis.