Subsequent Events (Textual) (Details) - Subsequent Event [Member]	Oct. 18, 2017 USD ($) ft² property	Oct. 16, 2017 USD ($) ft²	Oct. 02, 2017 USD ($) ft²	Oct. 17, 2017 USD ($)
DRA Austin Venture [Member]
Subsequent Event [Line Items]
Rentable Square Feet | ft²	1,164,496
Ownership percentage	50.00%
Number of Properties Sold | property	5
Sales Price	$ 333,300,000
Real estate properties encumbered, mortgage debt amount				$ 151,000,000
Cash proceeds after the payment of share of the debt, closing costs and prorations	86,400,000
Estimated gain on real estate venture transactions and equity in income of real estate ventures	$ 52,000,000
One Drexel Plaza and Drexel Square [Member]
Subsequent Event [Line Items]
Rentable Square Feet | ft²		282,709
Gross purchase value of assets		$ 35,000,000
Capitalized transaction costs		$ 2,700,000
Property improvement period		24 months
One Drexel Plaza and Drexel Square [Member] | Minimum [Member]
Subsequent Event [Line Items]
Property improvement obligation		$ 8,000,000
Four Points Building 3 [Member]
Subsequent Event [Line Items]
Net Rentable Square Feet | ft²			165,000
Percentage of property preleased by single tenant			100.00%
Lease agreement term			10 years
Estimated cost			$ 48,200,000
Estimated cost for existing land basis			$ 2,100,000