Significant Accounting Policies (Textual) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
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 is substantially complete (percent)	90.00%
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 is substantially complete (in years)	1 year
Reimbursement of rental revenue from tenants	$ 43.9	$ 26.2	$ 28.4
Required distribution of adjusted taxable income to qualify as a REIT	90.00%
Number of subsequent tax year (in years)	4 years
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
Minimum [Member] | Building [Member]
Property, Plant and Equipment [Line Items]
Useful life	24 years
Minimum [Member] | Furniture, Fixtures and Equipment [Member]
Property, Plant and Equipment [Line Items]
Useful life	3 years
Maximum [Member] | Building [Member]
Property, Plant and Equipment [Line Items]
Useful life	40 years
Maximum [Member] | Furniture, Fixtures and Equipment [Member]
Property, Plant and Equipment [Line Items]
Useful life	5 years