Segment Disclosure (Details 1) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011
Real Estate Acquisitions [Member]
Segment Reporting Information [Line Items]
Segment Reporting Information, Expenditures for Additions to Long-Lived Assets	$ 1,087,489	[1]	$ 663,483	[1]
Real Estate Acquisitions [Member] | Reportable Segment - Office Properties [Member]
Segment Reporting Information [Line Items]
Segment Reporting Information, Expenditures for Additions to Long-Lived Assets	1,087,489	[1]	663,483	[1]
Development and Redevelopment Properties and Undeveloped Land [Member]
Segment Reporting Information [Line Items]
Segment Reporting Information, Expenditures for Additions to Long-Lived Assets	144,477	[1]	32,198	[1]
Development and Redevelopment Properties and Undeveloped Land [Member] | Reportable Segment - Office Properties [Member]
Segment Reporting Information [Line Items]
Segment Reporting Information, Expenditures for Additions to Long-Lived Assets	144,477	[1]	32,198	[1]
Stabilized Portfolio [Member]
Segment Reporting Information [Line Items]
Segment Reporting Information, Expenditures for Additions to Long-Lived Assets	87,220	[1],[2]	72,449	[1],[2]
Stabilized Portfolio [Member] | Reportable Segment - Office Properties [Member]
Segment Reporting Information [Line Items]
Segment Reporting Information, Expenditures for Additions to Long-Lived Assets	86,089	[1],[2]	66,448	[1],[2]
Stabilized Portfolio [Member] | Non-Reportable Segment - Industrial Properties [Member]
Segment Reporting Information [Line Items]
Segment Reporting Information, Expenditures for Additions to Long-Lived Assets	$ 1,131	[1],[2]	$ 6,001	[1],[2]

[1] Total consolidated acquisitions and capital expenditures are equal to the same amounts disclosed for total segments. Amounts represent balances on an accrual basis.

[2] Includes expenditures for building improvements, tenant improvements, deferred leasing costs and acquisition-related intangible assets for our operating properties. Excludes acquisition-related intangible liabilities.