Real Estate Investments - Summary of Office Properties Sold (Parenthetical) (Details)			3 Months Ended				12 Months Ended
		Apr. 03, 2014 USD ($) ft² property Storey	Dec. 31, 2015 USD ($) ft² property Storey	Jun. 30, 2015 USD ($)	Mar. 31, 2015 USD ($)	Sep. 30, 2014 USD ($)	Dec. 31, 2015 USD ($) ft² property Storey		Dec. 31, 2014 USD ($) ft² property		Dec. 31, 2013 USD ($) ft² property Storey		Dec. 30, 2015 USD ($)	Oct. 17, 2014 ft² property	Jul. 31, 2014 ft² property	Oct. 16, 2013 USD ($)
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Rentable Square Feet | ft²			4,300,000				4,300,000
Number of Properties | property			179				179
Net gain (loss) on real estate venture transactions							$ 7,229,000		$ (417,000)		$ 29,604,000
Gain (Loss) on Sale							$ 20,496,000		$ 4,901,000		$ 0
Office Building [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Rentable Square Feet | ft²			2,649,926				2,649,926		645,273		2,472,036
Number of Properties | property			27				27		8		21
Gain (Loss) on Sale							$ 22,002,000	[1],[2]	$ 4,646,000	[3]	$ 29,233,000	[3]
Deferred gain			$ 500,000				$ 500,000
Encino Trace [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of floors of a building | Storey			4				4
Rentable Square Feet | ft²			320,000				320,000
Number of Properties | property			2				2
Encino Trace [Member] | Office Building [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Rentable Square Feet | ft²			320,000				320,000
Number of Properties | property			2				2
Gain (Loss) on Sale	[1],[4]						$ 2,008,000
Carlsbad, CA [Member] | Office Building [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Provision for impairment on assets held for sale			$ 6,300,000
Gain (Loss) on Sale							$ 0
100 Gateway Centre Parkway [Member] | Office Building [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Rentable Square Feet | ft²			74,991				74,991
Number of Properties | property			1				1
Provision for impairment on assets held for sale				$ 800,000
Gain (Loss) on Sale				$ 0			$ 0	[1],[5]
Lake Merritt Tower [Member] | Office Building [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Rentable Square Feet | ft²			204,336				204,336
Number of Properties | property			1				1
Provision for impairment on assets held for sale					$ 1,700,000
Gain (Loss) on Sale					$ 0		$ 0	[1],[6]
100, 101, 200, 300 and 301 Lindenwood Drive (the Valleybrooke Poperties) [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Provision for impairment on assets held for sale						$ 1,800,000
100, 101, 200, 300 and 301 Lindenwood Drive (the Valleybrooke Poperties) [Member] | Office Building [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Rentable Square Feet | ft²									279,934
Number of Properties | property									5
Gain (Loss) on Sale	[3],[7]								$ 203,000
Four Points Centre [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of floors of a building | Storey		3									3
Rentable Square Feet | ft²											192,396
Number of Properties | property		2									2
Four Points Centre [Member] | Office Building [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Rentable Square Feet | ft²									192,396
Number of Properties | property									2
Gain (Loss) on Sale	[3],[8]								$ (255,000)
Austin Joint Venture [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Rentable Square Feet | ft²														590,881	232,274
Number of Properties | property														7	2
Austin Joint Venture [Member] | Encino Trace [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Project expected cost							91,300,000
Non-Recourse Debt			$ 30,000,000				30,000,000
Amount Of Participant Contribution			1,800,000				1,800,000
Austin Joint Venture [Member] | Four Points Centre [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Non-Recourse Debt		$ 29,000,000
Austin Joint Venture [Member] | Four Points Centre [Member] | Office Building [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Rentable Square Feet | ft²		192,396
Brandywine Realty Trust [Member] | Austin Joint Venture [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Aggregate gross sales price of a joint venture																$ 330,000,000
Brandywine Realty Trust [Member] | Austin Joint Venture [Member] | Encino Trace [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Development costs													$ 76,700,000
Proceeds from Equity Method Investment, Dividends or Distributions							50,200,000
Capital contribution			23,300,000				23,300,000
Net gain (loss) on real estate venture transactions							2,000,000
Additional borrowing fund to tenant improvements and leasing commissions			29,700,000				29,700,000
Brandywine Realty Trust [Member] | Austin Joint Venture [Member] | Four Points Centre [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Amount Of Participant Contribution		$ 5,900,000
Proceeds from Equity Method Investment, Dividends or Distributions		34,400,000
Net gain (loss) on real estate venture transactions									$ (200,000)
Aggregate gross sales price of a joint venture		$ 41,500,000
DRA Advisors LLC [Member] | Austin Joint Venture [Member] | Encino Trace [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Working capital contribution			1,800,000				1,800,000
DRA Advisors LLC [Member] | Brandywine Realty Trust [Member] | Austin Joint Venture [Member] | Encino Trace [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Amount Of Participant Contribution			$ 25,100,000				$ 25,100,000

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

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

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

[4] On December 31, 2015, the Company contributed two newly constructed four-story, Class A office buildings, commonly known as “Encino Trace,” containing an aggregate of approximately 320,000 square feet in Austin, Texas to one of its existing real estate ventures (the “Austin Venture”) that the Company formed in 2013 with G&I VII Austin Office LLC, an investment vehicle advised by DRA Advisors LLC (“DRA”). When these two properties were contributed to the Austin Venture the Company had incurred a total of $76.7 million of development costs, representing the contribution value. The project is expected to cost $91.3 million with remaining costs fully funded by the Austin Venture. In conjunction with the contribution: (i) the Austin Venture obtained a $30.0 million mortgage loan; (ii) DRA contributed $25.1 million in net cash to the capital of the Austin Venture, including a $1.8 million working capital contribution; and (iii) the Austin Venture distributed $50.2 million to the Company and credited the Company with a $23.3 million capital contribution to the Austin Venture. In addition to the contribution of the properties, the Company also made a $1.8 million cash contribution to the Austin Venture for working capital. The Company recognized a $2.0 million gain on the contribution. Under the Encino Trace loan agreement the Austin Venture has the option, subject to certain leasing and loan-to-value requirements, to borrow an additional $29.7 million to fund tenant improvements and leasing commissions.

[5] The Company recorded an impairment loss of $0.8 million for 100 Gateway Centre Parkway during the second quarter of 2015. As such, there was no gain at disposition for this property.

[6] The Company recorded an impairment loss of $1.7 million for Lake Merritt Tower at March 31, 2015. As such, there was no gain at disposition for this property. Sales proceeds were deposited in escrow under Section 1031 of the Internal Revenue Code and applied to purchase the Broadmoor Austin portfolio. Refer to Broadmoor Austin Associates acquisition summary, above, for further details.

[7] During the third quarter of 2014, the Company recorded a $1.8 million impairment loss on these properties.

[8] On April 3, 2014, the Company contributed two three-story, Class A office buildings, commonly known as “Four Points Centre,” containing an aggregate of approximately 192,396 net rentable square feet in Austin, Texas to an existing real estate venture (the “Austin Venture”) that the Company formed in 2013 with G&I VII Austin Office LLC, an investment vehicle advised by DRA Advisors LLC (“DRA”). The Company contributed the properties to the Austin Venture at an agreed upon value of $41.5 million. In conjunction with the contribution: (i) the Austin Venture obtained a $29.0 million mortgage loan; (ii) DRA contributed $5.9 million in net cash to the capital of the Austin Venture; and (iii) the Austin Venture distributed $34.4 million to the Company and credited the Company with a $5.9 million capital contribution to the Austin Venture. The Company incurred a $0.2 million loss on the contribution, driven primarily by closing costs.