Real Estate Investments - Summary of Office Properties Sold (Details)		3 Months Ended		12 Months Ended
		Jun. 30, 2015 USD ($)	Mar. 31, 2015 USD ($)	Dec. 31, 2015 USD ($) ft² property		Dec. 31, 2014 USD ($) ft² property		Dec. 31, 2013 USD ($) ft² property		Apr. 03, 2014 property
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Properties | property				179
Rentable Square Feet | ft²				4,300,000
Net Proceeds on Sale				$ 247,228,000		$ 118,855,000		$ 423,480,000
Gain (Loss) on Sale				$ 20,496,000		$ 4,901,000		$ 0
Encino Trace [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Properties | property				2
Rentable Square Feet | ft²				320,000
Four Points Centre [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Properties | property								2		2
Rentable Square Feet | ft²								192,396
Office Building [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Properties | property				27		8		21
Rentable Square Feet | ft²				2,649,926		645,273		2,472,036
Sales Price				$ 374,600,000		$ 101,150,000		$ 491,825,000
Net Proceeds on Sale				340,098,000		113,024,000		418,547,000
Gain (Loss) on Sale				$ 22,002,000	[1],[2]	$ 4,646,000	[3]	$ 29,233,000	[3]
Office Building [Member] | Encino Trace [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Properties | property				2
Rentable Square Feet | ft²				320,000
Sales Price				$ 76,700,000
Net Proceeds on Sale				50,158,000
Gain (Loss) on Sale	[1],[4]			$ 2,008,000
Office Building [Member] | Laurel Corporate Center [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Properties | property				6
Rentable Square Feet | ft²				560,147
Sales Price				$ 56,500,000
Net Proceeds on Sale				56,253,000
Gain (Loss) on Sale	[1]			$ 2,901,000
Office Building [Member] | Carlsbad Properties [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Properties | property				3
Rentable Square Feet | ft²				196,075
Sales Price				$ 30,400,000
Net Proceeds on Sale				29,568,000
Gain (Loss) on Sale	[1],[5]			$ 0
Office Building [Member] | 751-761 Fifth Ave [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Properties | property				1
Rentable Square Feet | ft²				158,000
Sales Price				$ 4,600,000
Net Proceeds on Sale				4,245,000
Gain (Loss) on Sale	[1]			$ 894,000
Office Building [Member] | 1000 Howard Boulevard [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Properties | property				1
Rentable Square Feet | ft²				105,312
Sales Price				$ 16,500,000
Net Proceeds on Sale				15,780,000
Gain (Loss) on Sale	[1]			$ 4,828,000
Office Building [Member] | Bay Colony Office Park [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Properties | property				4
Rentable Square Feet | ft²				247,294
Sales Price				$ 37,500,000
Net Proceeds on Sale				36,386,000
Gain (Loss) on Sale	[1]			$ 269,000
Office Building [Member] | 741 First Avenue [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Properties | property				1
Rentable Square Feet | ft²				77,184
Sales Price				$ 4,900,000
Net Proceeds on Sale				4,640,000
Gain (Loss) on Sale	[1]			$ 372,000
Office Building [Member] | 100 Gateway Centre Parkway [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Properties | property				1
Rentable Square Feet | ft²				74,991
Sales Price				$ 4,100,000
Net Proceeds on Sale				3,911,000
Gain (Loss) on Sale		$ 0		$ 0	[1],[6]
Office Building [Member] | Christina & Delaware Corporate Centers [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Properties | property				5
Rentable Square Feet | ft²				485,182
Sales Price				$ 50,100,000
Net Proceeds on Sale				49,579,000
Gain (Loss) on Sale	[1]			$ 1,749,000
Office Building [Member] | Lake Merritt Tower [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Properties | property				1
Rentable Square Feet | ft²				204,336
Sales Price				$ 65,000,000
Net Proceeds on Sale				62,800,000
Gain (Loss) on Sale			$ 0	$ 0	[1],[7]
Office Building [Member] | 1000 Atrium Way / 457 Haddonfield Road (Atrium I / Libertyview) [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Properties | property				2
Rentable Square Feet | ft²				221,405
Sales Price				$ 28,300,000
Net Proceeds on Sale				26,778,000
Gain (Loss) on Sale	[1]			$ 8,981,000
Office Building [Member] | 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]
Number of Properties | property						5
Rentable Square Feet | ft²						279,934
Sales Price						$ 37,900,000
Net Proceeds on Sale						37,156,000
Gain (Loss) on Sale	[3],[8]					$ 203,000
Office Building [Member] | 1880 Campus Commons Drive (Campus Pointe) [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Properties | property						1
Rentable Square Feet | ft²						172,943
Sales Price						$ 42,500,000
Net Proceeds on Sale						41,476,000
Gain (Loss) on Sale	[3]					$ 4,698,000
Office Building [Member] | Four Points Centre [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Properties | property						2
Rentable Square Feet | ft²						192,396
Sales Price						$ 20,750,000
Net Proceeds on Sale						34,392,000
Gain (Loss) on Sale	[3],[9]					$ (255,000)
Office Building [Member] | 875 First Avenue [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Properties | property								1
Rentable Square Feet | ft²								50,000
Sales Price								$ 3,750,000
Net Proceeds on Sale								3,658,000
Gain (Loss) on Sale	[3]							$ 131,000
Office Building [Member] | 1336 Enterprise Drive [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Properties | property								1
Rentable Square Feet | ft²								39,330
Sales Price								$ 2,600,000
Net Proceeds on Sale								2,521,000
Gain (Loss) on Sale	[3]							$ 156,000
Office Building [Member] | Austin Properties/ DRA JV [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Properties | property								7
Rentable Square Feet | ft²								1,398,826
Sales Price								$ 330,000,000
Net Proceeds on Sale								266,340,000
Gain (Loss) on Sale	[3],[10]							$ 25,864,000
Office Building [Member] | 16870 West Bernardo [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Properties | property								1
Rentable Square Feet | ft²								68,708
Sales Price								$ 18,000,000
Net Proceeds on Sale								17,403,000
Gain (Loss) on Sale	[3]							$ (856,000)
Office Building [Member] | 100 Arrandale Boulevard [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Properties | property								1
Rentable Square Feet | ft²								34,931
Sales Price								$ 3,500,000
Net Proceeds on Sale								3,268,000
Gain (Loss) on Sale	[3]							$ (401,000)
Office Building [Member] | 1700 Paoli Pike (100 Applebrook) [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Properties | property								1
Rentable Square Feet | ft²								28,000
Sales Price								$ 2,675,000
Net Proceeds on Sale								2,544,000
Gain (Loss) on Sale	[3]							$ (444,000)
Office Building [Member] | Pacific View Plaza [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Properties | property								1
Rentable Square Feet | ft²								51,695
Sales Price								$ 10,300,000
Net Proceeds on Sale								9,950,000
Gain (Loss) on Sale	[3]							$ (521,000)
Office Building [Member] | Princeton Pike Corporate Center [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Properties | property								8
Rentable Square Feet | ft²								800,546
Sales Price								$ 121,000,000
Net Proceeds on Sale								112,863,000
Gain (Loss) on Sale	[3]							$ 5,304,000

[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 $6.3 million for the Carlsbad office properties during the fourth quarter of 2015. As such, there was no gain at disposition for this property.

[6] 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.

[7] 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.

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

[9] 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.

[10] The Austin portfolio was contributed to a newly formed real estate venture. For additional information, see Note 4, “Investments in Unconsolidated Ventures,” to our consolidated financial statements.