Acquisitions (Details Textuals) (USD $)	6 Months Ended
	Jun. 30, 2011
Aquisitions (Textuals) [Abstract]
Deferred revenue and acquisition-related intangible liabilities	$ 14,240,000
Purchase price for acquisitions of 601 108th Avenue N.E. Bellevue, WA as percentage of total purchase price of all acquisitions	52.10%
Deferred leasing costs and acquisition-related intangible assets	31,290,000
Assumed noncancellable ground lease expiration	Nov. 30, 2093
4.94%Mortgage Payable due April 15, 2015 [Member]
Aquisitions (Textuals) [Abstract]
Secured debt, net (Notes 5 and 12)	30,000,000
Initial premium on outstanding amount of secured debt	1,000,000
Leases Acquired In Place [Member]
Aquisitions (Textuals) [Abstract]
Deferred leasing costs and acquisition-related intangible assets	18,900,000
Weighted average amortization period in years	4.1
Leases Acquired in Place Above Market Adjustment [Member]
Aquisitions (Textuals) [Abstract]
Deferred leasing costs and acquisition-related intangible assets	6,600,000
Weighted average amortization period in years	4.5
Deferred Leasing Costs [Member]
Aquisitions (Textuals) [Abstract]
Deferred leasing costs and acquisition-related intangible assets	5,700,000
Weighted average amortization period in years	2.8
Leases Acquired In Place Below Market Adjustment [Member]
Aquisitions (Textuals) [Abstract]
Deferred revenue and acquisition-related intangible liabilities	9,000,000
Weighted average amortization period in years	4.3
Above Market Ground Lease [Member]
Aquisitions (Textuals) [Abstract]
Deferred revenue and acquisition-related intangible liabilities	5,200,000
Weighted average amortization period in years	82.5
All Other Acquisitions [Member]
Aquisitions (Textuals) [Abstract]
Deferred revenue and acquisition-related intangible liabilities	1,390,000
Initial premium on outstanding amount of secured debt	1,000,000
Purchase price of each acquisition as percentage of company's total assets	Less than 5%
Purchase price of each acquisition as percentage of company's total assets	Less than 10%
Deferred leasing costs and acquisition-related intangible assets	$ 17,500,000