Real Estate Investments - Summary of Office Properties Sold (Details)			3 Months Ended			12 Months Ended
		Feb. 02, 2017 property	Mar. 31, 2017 USD ($)	Jun. 30, 2015 USD ($)	Mar. 31, 2015 USD ($)	Dec. 31, 2017 USD ($) ft² property	Dec. 31, 2016 USD ($) ft² property		Dec. 31, 2015 USD ($) ft² property		Dec. 23, 2015 ft²
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Proceeds from the sale of properties						$ 171,860,000	$ 784,331,000		$ 247,228,000
Gain (Loss) on Sale						$ 31,657,000	$ 116,983,000		$ 20,496,000
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
Sales Price						$ 21,400,000
Proceeds from the sale of properties						11,200,000
Gain (Loss) on Sale						$ 6,500,000
2970 Market Street (Cira Square) [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Rentable Square Feet | ft²											862,692
Gain (Loss) on Sale			$ 500,000
Encino Trace [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Properties Sold | property									2
Rentable Square Feet | ft²									320,000
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
Rentable Square Feet | ft²							5,239,632		2,649,926
Sales Price							$ 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]
Office Properties [Member] | 11, 14, 15, 17 and 18 Campus Boulevard (Newtown Square) [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						252,802
Sales Price						$ 42,000,000
Proceeds from the sale of properties						40,459,000
Gain (Loss) on Sale	[1]					$ 19,642,000
Office Properties [Member] | 630 Allendale Road [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Properties Sold | property						1
Rentable Square Feet | ft²						150,000
Sales Price						$ 17,500,000
Proceeds from the sale of properties						16,580,000
Gain (Loss) on Sale	[1]					$ 3,605,000
Office Properties [Member] | 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]
Number of Properties Sold | property						4
Rentable Square Feet | ft²						134,794
Sales Price						$ 9,700,000
Proceeds from the sale of properties						8,650,000
Gain (Loss) on Sale	[1],[4]					$ (325,000)
Office Properties [Member] | 200, 210 & 220 Lake Drive East (Woodland Falls) [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Properties Sold | property						3
Rentable Square Feet | ft²						215,465
Sales Price						$ 19,000,000
Proceeds from the sale of properties						17,771,000
Gain (Loss) on Sale	[1],[5]					$ (249,000)
Office Properties [Member] | 11700, 11710, 11720 & 11740 Beltsville Drive (Calverton) [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Properties Sold | property						3
Rentable Square Feet | ft²						313,810
Sales Price						$ 9,000,000
Proceeds from the sale of properties						8,354,000
Gain (Loss) on Sale	[1],[6]					$ 0
Office Properties [Member] | 1200 & 1220 Concord Avenue (Concord Airport Plaza) [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Properties Sold | property		2				2
Rentable Square Feet | ft²						350,256
Sales Price						$ 33,100,000
Proceeds from the sale of properties						32,010,000
Gain (Loss) on Sale	[1],[7]					$ 551,000
Office Properties [Member] | 620, 640, 660 Allendale Road [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Properties Sold | property							3
Rentable Square Feet | ft²							156,669
Sales Price							$ 12,800,000
Proceeds from the sale of properties							12,014,000
Gain (Loss) on Sale	[1]						$ 2,382,000
Office Properties [Member] | 1120 Executive Plaza [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Properties Sold | property							1
Rentable Square Feet | ft²							95,183
Sales Price							$ 9,500,000
Proceeds from the sale of properties							9,241,000
Gain (Loss) on Sale	[1],[8]						$ (18,000)
Office Properties [Member] | 50 East Clementon Road [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Properties Sold | property							1
Rentable Square Feet | ft²							3,080
Sales Price							$ 1,100,000
Proceeds from the sale of properties							1,011,000
Gain (Loss) on Sale	[1]						$ (85,000)
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]
Number of Properties Sold | property							2
Rentable Square Feet | ft²							197,225
Sales Price							$ 44,500,000
Proceeds from the sale of properties							43,412,000
Gain (Loss) on Sale	[1],[9]						$ (752,000)
Office Properties [Member] | 2970 Market Street (Cira Square) [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Properties Sold | property							1
Rentable Square Feet | ft²							862,692
Sales Price							$ 354,000,000
Proceeds from the sale of properties							350,150,000
Gain (Loss) on Sale	[1]						$ 115,828,000
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
Rentable Square Feet | ft²							3,924,783
Sales Price							$ 398,100,000
Proceeds from the sale of properties							353,971,000
Gain (Loss) on Sale	[1],[10]						$ (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
Sales Price									$ 76,700,000
Proceeds from the sale of properties									50,158,000
Gain (Loss) on Sale	[2],[11]								$ 2,008,000
Office Properties [Member] | Laurel Corporate Center [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Properties Sold | property									6
Rentable Square Feet | ft²									560,147
Sales Price									$ 56,500,000
Proceeds from the sale of properties									56,253,000
Gain (Loss) on Sale	[2]								$ 2,901,000
Office Properties [Member] | Carlsbad Properties [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Properties Sold | property									3
Rentable Square Feet | ft²									196,075
Sales Price									$ 30,400,000
Proceeds from the sale of properties									29,568,000
Gain (Loss) on Sale	[2],[12]								$ 0
Office Properties [Member] | 751-761 Fifth Ave [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Properties Sold | property									1
Rentable Square Feet | ft²									158,000
Sales Price									$ 4,600,000
Proceeds from the sale of properties									4,245,000
Gain (Loss) on Sale	[2]								$ 894,000
Office Properties [Member] | 1000 Howard Boulevard [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Properties Sold | property									1
Rentable Square Feet | ft²									105,312
Sales Price									$ 16,500,000
Proceeds from the sale of properties									15,780,000
Gain (Loss) on Sale	[2]								$ 4,828,000
Office Properties [Member] | Bay Colony Office Park [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Properties Sold | property									4
Rentable Square Feet | ft²									247,294
Sales Price									$ 37,500,000
Proceeds from the sale of properties									36,386,000
Gain (Loss) on Sale	[2]								$ 269,000
Office Properties [Member] | 741 First Avenue [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Properties Sold | property									1
Rentable Square Feet | ft²									77,184
Sales Price									$ 4,900,000
Proceeds from the sale of properties									4,640,000
Gain (Loss) on Sale	[2]								$ 372,000
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
Rentable Square Feet | ft²									74,991
Sales Price									$ 4,100,000
Proceeds from the sale of properties									3,911,000
Gain (Loss) on Sale				$ 0					$ 0	[2],[13]
Office Properties [Member] | Christina & Delaware Corporate Centers [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									485,182
Sales Price									$ 50,100,000
Proceeds from the sale of properties									49,579,000
Gain (Loss) on Sale	[2]								$ 1,749,000
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
Rentable Square Feet | ft²									204,336
Sales Price									$ 65,000,000
Proceeds from the sale of properties									62,800,000
Gain (Loss) on Sale					$ 0				$ 0	[2],[14]
Office Properties [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 Sold | property									2
Rentable Square Feet | ft²									221,405
Sales Price									$ 28,300,000
Proceeds from the sale of properties									26,778,000
Gain (Loss) on Sale	[2]								$ 8,981,000
Retail Properties [Member] | 7000 Midlantic Drive [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Properties Sold | property						1
Rentable Square Feet | ft²						10,784
Sales Price						$ 8,200,000
Proceeds from the sale of properties						7,714,000
Gain (Loss) on Sale	[1]					$ 1,413,000
Mixed Use Properties [Member] | Philadelphia Marine Center (Marine Piers) [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Properties Sold | property						1
Rentable Square Feet | ft²						181,900
Sales Price						$ 21,400,000
Proceeds from the sale of properties						11,182,000
Gain (Loss) on Sale	[1],[15]					$ 6,498,000
Office, Retail and Mixed-use Properties [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Properties Sold | property						20
Rentable Square Feet | ft²						1,609,811
Sales Price						$ 159,900,000
Proceeds from the sale of properties						142,720,000
Gain (Loss) on Sale	[1]					$ 31,135,000

[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] 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] As of June 30, 2016, the Company determined that the sale of the property was probable and classified this property as held for sale in accordance with applicable accounting standards for long lived assets. At such date, the carrying value of the property exceeded the fair value less the anticipated costs of sale. As a result, the Company recognized a provision for impairment totaling approximately $1.8 million during the three-month period ended June 30, 2016. The fair value measurement was based on the pricing in the purchase and sale agreement for the sale of the property. As the pricing in the purchase and sale agreement is unobservable, the Company determined that the inputs utilized to determine fair value for this property falls within Level 3 in accordance with the fair value hierarchy established by Accounting Standards Codification (ASC) Topic 820, "Fair Value Measurements and Disclosures.” The loss on sale represents additional closing costs recognized at closing.

[9] 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.

[10] 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.

[11] 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.

[12] 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.

[13] 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.

[14] 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.

[15] On March 15, 2017, the Company sold its sublease interest in the Piers at Penn’s Landing (the “Marine Piers”), which includes leasehold improvements containing 181,900 net rentable square feet, and a marina, located in Philadelphia, Pennsylvania for an aggregate sales price of $21.4 million. On the closing date, the buyer paid $12.0 million in cash. The $9.4 million balance of the purchase is due on (a) January 31, 2020, in the event that the tenant at the Marine Piers does not exercise an option it holds to extend the term of the sublease or (b) January 15, 2024, in the event that the tenant does exercise the option to extend the term of the sublease. In accordance with ASC 360-20, “Real Estate Sales,” the Company determined that it is appropriate to account for the sales transaction under the cost recovery method. The Company received cash proceeds of $11.2 million, after closing costs and prorations, and the net book value of the Marine Piers was $4.7 million, resulting in a gain on sale of $6.5 million. The remaining gain on sale of $9.4 million will be recognized on the second purchase price installment date. Prior to its sale, the Marine Piers had been classified as mixed-use within the Company’s property count.