Real Estate Investments - Summary of Land Parcels Sold (Details) $ in Thousands		12 Months Ended
		Dec. 31, 2016 USD ($) a Parcel		Dec. 31, 2015 USD ($) a Parcel		Dec. 31, 2014 USD ($) a Parcel		Jan. 15, 2016 Parcel
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Proceeds from the sale of properties		$ 784,331		$ 247,228		$ 118,855
Gain (Loss) on Sale		$ 116,983		$ 20,496		$ 4,901
Greenhills Land [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Parcels | Parcel								120
Land [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Parcels | Parcel		3		4		2
Acreage of land | a		122.9		14.7		22.1
Sales Price		$ 14,938		$ 22,300		$ 5,120
Proceeds from the sale of properties		14,532		21,088		4,855
Gain (Loss) on Sale		$ 9,232	[1]	$ 3,019	[2]	$ 1,184	[3]
Land [Member] | Oakland Lot B [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Parcels | Parcel		1
Acreage of land | a		0.9
Sales Price		$ 13,750
Proceeds from the sale of properties		13,411
Gain (Loss) on Sale	[1]	$ 9,039
Land [Member] | Highlands Land [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Parcels | Parcel		1
Acreage of land | a		2.0
Sales Price		$ 288
Proceeds from the sale of properties		284
Gain (Loss) on Sale	[1]	$ 193
Land [Member] | Greenhills Land [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Parcels | Parcel		1
Acreage of land | a		120.0
Sales Price		$ 900
Proceeds from the sale of properties		837
Gain (Loss) on Sale	[1],[4]	$ 0
Land [Member] | Two Christina Centre [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Parcels | Parcel				1
Acreage of land | a				1.6
Sales Price				$ 6,500
Proceeds from the sale of properties				5,986
Gain (Loss) on Sale	[2],[5]			$ 0
Land [Member] | A7000 Midlantic [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Parcels | Parcel				1
Acreage of land | a				3.5
Sales Price				$ 2,200
Proceeds from the sale of properties				1,742
Gain (Loss) on Sale	[2]			$ (169)
Land [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 Parcels | Parcel				1
Acreage of land | a				8.6
Sales Price				$ 2,500
Proceeds from the sale of properties				2,344
Gain (Loss) on Sale	[2]			$ 71
Land [Member] | Two Kaiser Plaza [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Parcels | Parcel				1
Acreage of land | a				1.0
Sales Price				$ 11,100
Proceeds from the sale of properties				11,016
Gain (Loss) on Sale	[2]			$ 3,117
Land [Member] | West Point II Land [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Parcels | Parcel						1
Acreage of land | a						5.3
Sales Price						$ 1,600
Proceeds from the sale of properties						1,505
Gain (Loss) on Sale	[3]					$ 12
Land [Member] | Rob Roy - Land [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Parcels | Parcel						1
Acreage of land | a						16.8
Sales Price						$ 3,520
Proceeds from the sale of properties						3,350
Gain (Loss) on Sale	[3]					$ 1,172

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

[2] Gain/(Loss) on sale includes closing and other transaction related costs.

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

[4] The carrying value of the land exceeded the fair value less the anticipated costs of sale as of December 31, 2015, therefore the Company recognized an impairment loss of $0.3 million during the three-month period ended December 31, 2015. There was no gain or loss recognized on the sale during 2016.

[5] The Company recorded an impairment loss of $0.3 million for Two Christina Centre during the fourth quarter of 2015. As such, there was no gain/(loss) at disposition for this land parcel.