Acquisitions (Details) (USD $)				0 Months Ended					0 Months Ended
	Sep. 30, 2014 $500 Million Unsecured Line of Credit [Member] Unsecured Debt [Member]	Sep. 30, 2014 Mortgage Loan Maturing September 1, 2021 [Member] Secured Debt [Member]	Jun. 27, 2014 Mortgage Loan Maturing September 1, 2021 [Member] Secured Debt [Member]	Jun. 27, 2014 The 5 Wall Street Building [Member]		Jun. 27, 2014 The 5 Wall Street Building [Member] sqft		Jun. 27, 2014 The 5 Wall Street Building [Member] Mortgage Loan Maturing September 1, 2021 [Member] Secured Debt [Member]	Aug. 28, 2014 1155 Perimeter Center West [Member]	Aug. 28, 2014 1155 Perimeter Center West [Member] sqft
Business Acquisition [Line Items]
Ownership of properties acquired						100.00%				100.00%
Borrowing capacity of line of credit	$ 500,000,000
Area of property						181,680	[1]			376,694
Percentage Leased as of Acquisition						100.00%	[1]			100.00%
Purchase Price (in millions)				62,500,000	[1]				80,800,000
Face amount of debt instrument	500,000,000	35,000,000	35,000,000.0					35,000,000
Mortgage loan assumed								$ 33,700,000

[1] The purchase included the assumption of a $35.0 million mortgage loan with a principal balance outstanding at acquisition of approximately $33.7 million, maturing on September 1, 2021. See further detail on the mortgage loan in Note 4.