Real Estate Investments - Summary of Office Properties Sold (Parenthetical) (Details)			3 Months Ended						12 Months Ended
		Feb. 04, 2016 property	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2015 USD ($) ft² Storey	Jun. 30, 2015 USD ($)	Mar. 31, 2015 USD ($)	Sep. 30, 2014 USD ($)	Dec. 31, 2016 USD ($) ft² property		Dec. 31, 2015 USD ($) ft² property Storey		Dec. 31, 2014 USD ($) ft² property Storey		Oct. 17, 2014 ft²	Jul. 31, 2014 ft²	Apr. 03, 2014 USD ($) ft² Storey	Oct. 16, 2013 USD ($)
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Provision for impairment			$ 1,800,000						$ 40,517,000		$ 82,208,000		$ 1,765,000
Impairment of properties held for use									7,300,000
Gain (loss) on real estate venture transactions									20,000,000		7,229,000		(417,000)
Gain (Loss) on Sale									$ 116,983,000		$ 20,496,000		$ 4,901,000
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
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
Encino Trace [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Properties Sold | property											2
Number of floors of a building | Storey					4						4
Rentable Square Feet | ft²					320,000						320,000
Encino Trace [Member] | Austin Joint Venture [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
Encino Trace [Member] | Brandywine Realty Trust [Member] | Austin Joint Venture [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Development costs					76,700,000						76,700,000
Proceeds from Equity Method Investment, Dividends or Distributions											50,200,000
Capital contribution					23,300,000						23,300,000
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
Encino Trace [Member] | DRA Advisors LLC [Member] | Austin Joint Venture [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
Encino Trace [Member] | DRA Advisors LLC [Member] | Brandywine Realty Trust [Member] | Austin Joint Venture [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
100, 101, 200, 300 and 301 Lindenwood Drive (the Valleybrooke Properties) [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
11305 Four Points Drive (Four Points Centre) [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Properties Sold | property													2
Number of floors of a building | Storey													3				3
Rentable Square Feet | ft²													192,396
11305 Four Points Drive (Four Points Centre) [Member] | Austin Joint Venture [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Non-Recourse Debt																	$ 29,000,000
11305 Four Points Drive (Four Points Centre) [Member] | Brandywine Realty Trust [Member] | Austin Joint Venture [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Amount Of Participant Contribution																	5,900,000
Gain (loss) on real estate venture transactions													$ (200,000)
Additional borrowing fund to tenant improvements and leasing commissions													$ 34,400,000
Aggregate gross sales price of a joint venture																	$ 41,500,000
Office Properties [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Properties Sold | property									66		27		8
Rentable Square Feet | ft²					2,649,926				5,239,632		2,649,926		645,273
Gain (Loss) on Sale									$ 116,983,000	[1]	$ 22,002,000	[2],[3]	$ 4,646,000	[4]
Deferred gain					$ 500,000						$ 500,000
Office Properties [Member] | 196/198 Van Buren Street (Herndon Metro Plaza I&II) [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Impairment of properties held for use				$ 7,400,000
Number of Properties Sold | property									2
Rentable Square Feet | ft²									197,225
Gain (Loss) on Sale	[1],[5]								$ (752,000)
Office Properties [Member] | Och Ziff Portfolio [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Properties Sold | property		58							58	[6]
Provision for impairment on assets held for sale					$ 45,400,000
Rentable Square Feet | ft²	[6]								3,924,783
Gain (Loss) on Sale	[1],[6],[7]								$ (372,000)
Office Properties [Member] | Encino Trace [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Properties Sold | property											2
Rentable Square Feet | ft²					320,000						320,000
Gain (Loss) on Sale	[2],[8]										$ 2,008,000
Office Properties [Member] | Carlsbad, CA [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
Office Properties [Member] | 100 Gateway Centre Parkway [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Properties Sold | property											1
Provision for impairment on assets held for sale						$ 800,000
Rentable Square Feet | ft²					74,991						74,991
Gain (Loss) on Sale						$ 0					$ 0	[2],[9]
Office Properties [Member] | Lake Merritt Tower [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Properties Sold | property											1
Provision for impairment on assets held for sale							$ 1,700,000
Rentable Square Feet | ft²					204,336						204,336
Gain (Loss) on Sale							$ 0				$ 0	[2],[10]
Office Properties [Member] | 100, 101, 200, 300 and 301 Lindenwood Drive (the Valleybrooke Properties) [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Properties Sold | property													5
Rentable Square Feet | ft²													279,934
Gain (Loss) on Sale	[4],[11]												$ 203,000
Office Properties [Member] | 11305 Four Points Drive (Four Points Centre) [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Properties Sold | property													2
Rentable Square Feet | ft²													192,396
Gain (Loss) on Sale	[4],[12]												$ (255,000)
Office Properties [Member] | 11305 Four Points Drive (Four Points Centre) [Member] | Austin Joint Venture [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Rentable Square Feet | ft²																	192,396

[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.

[5] During the three-month period ended March 31, 2016, the Company recognized a provision for impairment totaling approximately $7.4 million on the properties. See “Held for Use Impairment” section below. The loss on sale primarily relates to additional closing costs recognized at closing.

[6] Exhibit 99.2 to the Company’s Current Report on Form 8-K filed on February 10, 2016 contains a complete list of the 58 properties disposed of in the transactions with Och-Ziff Capital Management Group LLC. See Note 4, "Investment in Unconsolidated Real Estate Ventures," for further details of the transactions.

[7] During the three-month period ended December 31, 2015, the Company recognized a provision for impairment totaling approximately $45.4 million. The loss on sale represents additional closing costs recognized at closing.

[8] 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.

[9] 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.

[10] 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.

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

[12] 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.