Segment Disclosure (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012 segment		Dec. 31, 2011 segment		Dec. 31, 2010	Dec. 31, 2009
Segment Reporting Information [Line Items]
Number of Reportable Segments	1		1
Number of Non-Reportable Segments			1
Assets:
Land, buildings and improvement, net	$ 3,191,225		$ 2,625,381
Undeveloped land and construction in progress	809,654		430,806
TOTAL ASSETS	4,616,084		3,446,795
Reconciliation to Consolidated Assets:
Total assets allocated to segments	4,616,084		3,446,795
Other unallocated asset:
Cash and cash equivalents	16,700		4,777		14,840	9,883
Restricted cash	247,544		358
Marketable securities	7,435		5,691
Deferred financing costs, net	18,971		18,368
Prepaid expenses and other assets, net	9,949		12,199
TOTAL ASSETS	4,616,084		3,446,795
Operating Segments [Member]
Assets:
TOTAL ASSETS	4,315,485	[1]	3,405,402	[1]
Reconciliation to Consolidated Assets:
Total assets allocated to segments	4,315,485	[1]	3,405,402	[1]
Other unallocated asset:
TOTAL ASSETS	4,315,485	[1]	3,405,402	[1]
Reportable Segment - Office Properties [Member]
Assets:
Land, buildings and improvement, net	3,191,225		2,480,338
Undeveloped land and construction in progress	809,654		430,806
TOTAL ASSETS	4,315,485	[1]	3,248,661	[1]
Reconciliation to Consolidated Assets:
Total assets allocated to segments	4,315,485	[1]	3,248,661	[1]
Other unallocated asset:
TOTAL ASSETS	4,315,485	[1]	3,248,661	[1]
Non-Reportable Segment - Industrial Properties [Member]
Assets:
Land, buildings and improvement, net	0		145,043
TOTAL ASSETS	0	[1]	156,741	[1]
Reconciliation to Consolidated Assets:
Total assets allocated to segments	0	[1]	156,741	[1]
Other unallocated asset:
TOTAL ASSETS	$ 0	[1]	$ 156,741	[1]

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