Significant Accounting Policies (Details Textual) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Significant Accounting Policies (Textual) [Abstract]
Period in which the company capitalizes interest, real estate taxes and certain operating expenses on the unoccupied portion of recently completed properties, beginning with the date it receives its certificate of occupancy	95.00%
Period in which the company capitalizes interest, real estate taxes and certain operating expenses on the unoccupied portion of recently completed properties, beginning with the date it receives its certificate of occupancy	1 year
Reimbursement of rental revenue from tenants	$ 26.9	$ 25.5	$ 25.4
Period of delinquency when the Company generally reserves accounts receivable for specific tenants on rental payments and reimbursements	90 days or more
Period of delinquency when the Company also reserves accounts receivable when it deems that collectability is highly questionable	less than 90 days
Required distribution of adjusted taxable income to qualify as a REIT	90.00%
Original maturities of securities and repurchase agreements included in highly-liquid money market instruments	three months or less
Money market mutual funds and treasury bills included in highly-liquid money market instruments	30 days or less
Building [Member]
Property, Plant and Equipment [Line Items]
Maximum useful life	40
Minimum useful life	24
Furniture, Fixtures and Equipment [Member]
Property, Plant and Equipment [Line Items]
Maximum useful life	5
Minimum useful life	3