Real Estate Investments - Summary of Properties Classified as Held for Sale Included in Continuing Operations (Details) - USD ($) $ in Thousands		Sep. 30, 2017	Dec. 31, 2016
ASSETS HELD FOR SALE
Total assets held for sale, net		$ 31,125	$ 41,718
LIABILITIES HELD FOR SALE
Total liabilities held for sale		269	$ 81
Held for Sale Properties Included in Continuing Operations [Member]
ASSETS HELD FOR SALE
Operating properties		47,291
Accumulated depreciation		(16,191)
Operating real estate investments, net		31,100
Construction-in-progress		25
Total assets held for sale, net		31,125
LIABILITIES HELD FOR SALE
Other liabilities		269
Total liabilities held for sale		269
Held for Sale Properties Included in Continuing Operations [Member] | 630 Allendale Road - Office [Member]
ASSETS HELD FOR SALE
Operating properties	[1]	19,482
Accumulated depreciation	[1]	(6,985)
Operating real estate investments, net	[1]	12,497
Construction-in-progress	[1]	0
Total assets held for sale, net	[1]	12,497
LIABILITIES HELD FOR SALE
Other liabilities	[1]	35
Total liabilities held for sale	[1]	35
Held for Sale Properties Included in Continuing Operations [Member] | Newtown Square - Office [Member]
ASSETS HELD FOR SALE
Operating properties	[2]	27,809
Accumulated depreciation	[2]	(9,206)
Operating real estate investments, net	[2]	18,603
Construction-in-progress	[2]	25
Total assets held for sale, net	[2]	18,628
LIABILITIES HELD FOR SALE
Other liabilities	[2]	234
Total liabilities held for sale	[2]	$ 234

[1] Office property containing 150,000 rentable square feet located in the Pennsylvania Suburbs segment.

[2] Five office properties containing 252,802 rentable square feet located in the Pennsylvania Suburbs segment.