Real Estate Investments - Summary of Office Properties Sold (Parenthetical) (Details)						3 Months Ended								12 Months Ended
		Oct. 18, 2017 USD ($) ft² property	Oct. 17, 2017 property	Mar. 31, 2017 USD ($) property segment	Feb. 02, 2017 property	Dec. 31, 2017 USD ($) ft² property	Mar. 31, 2017 USD ($) property segment	Dec. 31, 2016 USD ($) ft² property	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2015 USD ($) ft² Storey	Jun. 30, 2015 USD ($)	Mar. 31, 2015 USD ($)	Dec. 31, 2017 USD ($) ft² property	Dec. 31, 2016 USD ($) ft² property		Dec. 31, 2015 USD ($) ft² property Storey		Oct. 17, 2014 ft² property	Jul. 31, 2014 ft² property
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Properties | property						93								93
Provision for impairment									$ 1,800,000					$ 3,057,000	$ 40,517,000		$ 82,208,000
Proceeds from the sale of properties														171,860,000	784,331,000		247,228,000
Assets held for sale, net						$ 392,000		$ 41,718,000						392,000	41,718,000
Gain (Loss) on Sale														31,657,000	116,983,000		20,496,000
Proceeds after closing costs and prorations														0	0		62,800,000
Gain (loss) on real estate venture transactions														$ 80,526,000	$ 20,000,000		$ 7,229,000
Austin Joint Venture [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Properties | property																			7	2
Rentable Square Feet | ft²		1,164,496				1,570,123								1,570,123					590,881	232,274
Sales Price		$ 333,250,000
Number of Properties Sold | property		8	7			8
Gain (loss) on real estate venture transactions		$ 40,092,000				$ 52,200,000
Office Properties [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Properties | property						81								81
Rentable Square Feet | ft²								5,239,632			2,649,926				5,239,632		2,649,926
Sales Price								$ 820,000,000			$ 374,600,000				$ 820,000,000		$ 374,600,000
Proceeds from the sale of properties															769,799,000		340,098,000
Gain (Loss) on Sale															$ 116,983,000	[1]	22,002,000	[2],[3]
Deferred gain on sale											500,000						$ 500,000
Number of Properties Sold | property															66		27
Deferred gain											500,000						$ 500,000
Other Segments
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Properties				4			4	3							3
Provision for impairment						$ 0
Carrying value of real estate prior to impairment						$ 10,200,000		$ 25,800,000						$ 10,200,000	$ 25,800,000
Aggregate carrying value								18,500,000							$ 18,500,000
Impairment Hold Period				10 years											10 years
Residual Capitalization Rates				9.00%											8.75%
Discount Rates				9.25%											9.00%
Impairment of properties held for use				$ 1,000,000											$ 7,300,000
Two, Four A, Four B and Five Eves Drive (Evesham Corporate Center) [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Provision for impairment							$ 1,000,000
Carrying value of real estate prior to impairment				10,200,000			10,200,000
Aggregate carrying value				$ 9,200,000			$ 9,200,000
Impairment Hold Period							10 years
Residual Capitalization Rates							9.00%
Discount Rates							9.25%
Two, Four A, Four B and Five Eves Drive (Evesham Corporate Center) [Member] | Office Properties [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Rentable Square Feet | ft²						134,794								134,794
Sales Price						$ 9,700,000								$ 9,700,000
Proceeds from the sale of properties														8,650,000
Gain (Loss) on Sale	[1],[4]													$ (325,000)
Number of Properties Sold | property														4
Two, Four A, Four B and Five Eves Drive (Evesham Corporate Center) [Member] | Other Segments
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Properties | property				4			4
200, 210 & 220 Lake Drive East (Woodland Falls) [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Provision for impairment								7,300,000
Carrying value of real estate prior to impairment								25,800,000							25,800,000
Aggregate carrying value								$ 18,500,000							$ 18,500,000
Impairment Hold Period								10 years
Residual Capitalization Rates								8.75%
Discount Rates								9.00%
200, 210 & 220 Lake Drive East (Woodland Falls) [Member] | Office Properties [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Rentable Square Feet | ft²						215,465								215,465
Sales Price						$ 19,000,000								$ 19,000,000
Proceeds from the sale of properties														17,771,000
Gain (Loss) on Sale	[1],[5]													$ (249,000)
Number of Properties Sold | property														3
200, 210 & 220 Lake Drive East (Woodland Falls) [Member] | Other Segments
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Properties | property								3							3
Philadelphia Marine Center (Marine Piers) [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Rentable Square Feet | ft²						181,900								181,900
Sales Price						$ 21,400,000								$ 21,400,000
Payment by cash on purchases														12,000,000
Balance payment on purchases						9,400,000								$ 9,400,000
Date of second installment payment														Jan. 31, 2020
Proceeds from the sale of properties														$ 11,200,000
Assets held for sale, net						4,700,000								4,700,000
Gain (Loss) on Sale														6,500,000
Deferred gain on sale						9,400,000								9,400,000
Deferred gain						$ 9,400,000								$ 9,400,000
11700, 11710, 11720 & 11740 Beltsville Drive (Calverton) [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Provision for impairment on assets held for sale							$ 1,700,000	$ 3,000,000
Purchase price reduction							$ 1,700,000
11700, 11710, 11720 & 11740 Beltsville Drive (Calverton) [Member] | Office Properties [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Rentable Square Feet | ft²						313,810								313,810
Sales Price						$ 9,000,000								$ 9,000,000
Proceeds from the sale of properties														8,354,000
Gain (Loss) on Sale	[1],[6]													$ 0
Number of Properties Sold | property														3
1200 & 1220 Concord Avenue (Concord Airport Plaza) [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Provision for impairment on assets held for sale														$ 32,000
Proceeds after closing costs and prorations								$ 11,500
1200 & 1220 Concord Avenue (Concord Airport Plaza) [Member] | Office Properties [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Rentable Square Feet | ft²						350,256								350,256
Sales Price						$ 33,100,000								$ 33,100,000
Proceeds from the sale of properties														32,010,000
Gain (Loss) on Sale	[1],[7]													$ 551,000
Number of Properties Sold | property					2									2
196/198 Van Buren Street (Herndon Metro Plaza I&II) [Member] | Office Properties [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Rentable Square Feet | ft²								197,225							197,225
Sales Price								$ 44,500,000							$ 44,500,000
Proceeds from the sale of properties															43,412,000
Gain (Loss) on Sale	[1],[8]														$ (752,000)
Impairment of properties held for use										$ 7,400,000
Number of Properties Sold | property															2
Och Ziff Portfolio [Member] | Office Properties [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Rentable Square Feet | ft²								3,924,783							3,924,783
Sales Price								$ 398,100,000							$ 398,100,000
Proceeds from the sale of properties															353,971,000
Gain (Loss) on Sale	[1],[9]														$ (372,000)
Provision for impairment on assets held for sale											$ 45,400,000
Number of Properties Sold | property															58
Encino Trace [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 floors of a building | Storey											4						4
Number of Properties Sold | property																	2
Encino Trace [Member] | Austin Joint Venture [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
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
Encino Trace [Member] | Office Properties [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Rentable Square Feet | ft²											320,000						320,000
Sales Price											$ 76,700,000						$ 76,700,000
Proceeds from the sale of properties																	50,158,000
Gain (Loss) on Sale	[2],[10]																$ 2,008,000
Number of Properties Sold | property																	2
Carlsbad, CA [Member] | Office Properties [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Gain (Loss) on Sale											0
Provision for impairment on assets held for sale											$ 6,300,000
100 Gateway Centre Parkway [Member] | Office Properties [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Rentable Square Feet | ft²											74,991						74,991
Sales Price											$ 4,100,000						$ 4,100,000
Proceeds from the sale of properties																	3,911,000
Gain (Loss) on Sale												$ 0					$ 0	[2],[11]
Provision for impairment on assets held for sale												$ 800,000
Number of Properties Sold | property																	1
Lake Merritt Tower [Member] | Office Properties [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Rentable Square Feet | ft²											204,336						204,336
Sales Price											$ 65,000,000						$ 65,000,000
Proceeds from the sale of properties																	62,800,000
Gain (Loss) on Sale													$ 0				$ 0	[2],[12]
Provision for impairment on assets held for sale													$ 1,700,000
Number of Properties Sold | property																	1

[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] As of March 31, 2017, the Company evaluated the recoverability of the carrying value of its properties that triggered assessment under the undiscounted cash flow model. Based on the Company’s evaluation, it was determined that due to the reduction in the Company’s intended hold period of four properties located in the Other segment, the Company would not recover the carrying values of these properties. Accordingly, the Company recorded impairment charges on these properties of $1.0 million at March 31, 2017, which reduced the aggregate carrying values of the properties from $10.2 million to their estimated fair value of $9.2 million. The Company measured these impairments based on a discounted cash flow analysis, using a hold period of 10 years and residual capitalization rates and discount rates of 9.00% and 9.25%, respectively. The results were comparable to indicative pricing in the market. The assumptions used to determine fair value under the income approach are Level 3 inputs in accordance with the fair value hierarchy established by Accounting Standards Codification (ASC) Topic 820, “Fair Value Measurements and Disclosures.” The loss on sale in the table above represents additional closing costs.

[5] As of December 31, 2016, the Company evaluated the recoverability of the carrying value of its properties that triggered assessment under the undiscounted cash flow model. Based on the Company’s evaluation, it was determined that due to the reduction in the Company’s intended hold period of three properties located in the Other segment, the Company would not recover the carrying values of these properties. Accordingly, the Company recorded impairment charges on these properties of $7.3 million at December 31, 2016, reducing the aggregate carrying values of the properties from $25.8 million to their estimated fair value of $18.5 million. The Company measured these impairments based on a discounted cash flow analysis, using a hold period of 10 years and residual capitalization rates and discount rates of 8.75% and 9.00%, respectively. The results were comparable to indicative pricing in the market. The assumptions used to determine fair value under the income approach are Level 3 inputs in accordance with the fair value hierarchy established by Accounting Standards Codification (ASC) Topic 820, “Fair Value Measurements and Disclosures.” The loss on sale in the table above represents additional closing costs.

[6] During the fourth quarter of 2016, the Company recognized a $3.0 million impairment related to these properties. During the first quarter of 2017, there was a price reduction of $1.7 million under the agreement of sale and an additional impairment of $1.7 million was recognized.

[7] During the fourth quarter of 2016, the Company recognized an $11.5 million impairment related to these properties. This sale was designated as a like-kind exchange under Section 1031 of the Internal Revenue Code (“IRC”) and, as such, the proceeds, totaling $32.0 million after closing costs and prorations, were deposited with a Qualified Intermediary, as defined under the IRC. The proceeds received at closing were recorded as “Other assets” in the Company’s consolidated balance sheets. During the third quarter of 2017, the Company acquired 3000 Market Street in Philadelphia, Pennsylvania using the full balance of the Section 1031 proceeds. See “Acquisition” section above.

[8] 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.

[9] 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.

[10] 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. 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.

[11] 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.

[12] The Company recorded an impairment loss of $1.7 million for Lake Merritt Tower on 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.