Subsequent Events (Textual) (Details)			12 Months Ended
	Feb. 02, 2017 USD ($) ft² property	Jan. 31, 2017 USD ($) property	Dec. 31, 2016 USD ($) ft² property		Dec. 31, 2015 USD ($) ft² property		Dec. 31, 2014 USD ($) ft² property		Jan. 30, 2017 USD ($) a	Jan. 10, 2017 shares
Subsequent Event [Line Items]
Gain (Loss) on Sale			$ 116,983,000		$ 20,496,000		$ 4,901,000
Net gain (loss) on real estate venture transactions			$ 20,000,000		$ 7,229,000		$ (417,000)
Subsequent Event [Member]
Subsequent Event [Line Items]
Shares Authorized For Equity Offering Program | shares										16,000,000
TB-BDN Plymouth Apartments [Member] | Subsequent Event [Member]
Subsequent Event [Line Items]
Gross sales price		$ 100,500,000
Sale of ownership interest percentage		50.00%
Guarantee obligations cancelled		$ 3,200,000
Equity in income (loss) of Real Estate Ventures		23,300,000
Net gain (loss) on real estate venture transactions		14,600,000
Cash proceeds after the payment of share of the debt and closing costs		27,200,000
TB-BDN Plymouth Apartments [Member] | Subsequent Event [Member] | Pennsylvania [Member]
Subsequent Event [Line Items]
Construction Loan		$ 54,000,000
TB-BDN Plymouth Apartments [Member] | Subsequent Event [Member] | Multi-Family Complex [Member]
Subsequent Event [Line Items]
Number of Property units | property		398
Garza Land Sale [Member] | Subsequent Event [Member]
Subsequent Event [Line Items]
Area of land under agreement to sell | a									1.7
Sales price of properties sold									$ 3,500,000
Office Properties [Member]
Subsequent Event [Line Items]
Number of properties | property			66		27		8
Square feet of office space leased | ft²			5,239,632		2,649,926		645,273
Gross sales price			$ 820,000,000		$ 374,600,000		$ 101,150,000
Gain (Loss) on Sale			$ 116,983,000	[1]	$ 22,002,000	[2],[3]	$ 4,646,000	[4]
Office Properties [Member] | 1200 and 1220 Concord Avenue in Concord [Member] | Subsequent Event [Member]
Subsequent Event [Line Items]
Number of properties | property	2
Square feet of office space leased | ft²	350,256
Gross sales price	$ 33,100,000
Gain (Loss) on Sale	500,000
Proceeds after closing costs and prorations	$ 32,000,000

[1] Gain/(Loss) on Sale is net of closing and other transaction related costs.

[2] Gain on Sale is net of closing and other transaction related costs.

[3] Total gain on sale does not include a deferred gain of $0.5 million related to a prior sale.

[4] Gain/(Loss) on sale is net of closing and other transaction related costs.