Real Estate Investments - Purchase Price Allocation (Details) - USD ($) $ in Thousands		Oct. 13, 2017	Jul. 28, 2017	Jun. 22, 2015
One Drexel Plaza [Member]
Business Acquisition [Line Items]
Building, land and improvements		$ 30,583
Construction-in-progress		672
Intangible assets acquired	[1]	10,575
Below market lease liabilities assumed	[2]	(4,055)
Total purchase price		$ 37,775
3000 Market Street [Member]
Business Acquisition [Line Items]
Building, land and improvements			$ 32,004
Intangible assets acquired	[3]		2,562
Below market lease liabilities assumed	[4]		(1,818)
Total purchase price			$ 32,748
Broadmoor Austin Associates [Member]
Business Acquisition [Line Items]
Building, land and improvements				$ 163,271
Land inventory				6,045
Intangible assets acquired	[5]			50,637
Below market lease liabilities assumed	[6]			(8,600)
Total purchase price				211,353
Return of existing equity method investment				(66,324)
Gain on remeasurement				(758)
Net working capital assumed				(450)
Total cash payment at settlement				$ 143,821

[1] Weighted average amortization period of 7.9 years.

[2] Weighted average amortization period of 7.0 years.

[3] Weighted average amortization period of 5.9 years.

[4] Weighted average amortization period of 6.0 years.

[5] Weighted average amortization period of 4.0 years.

[6] Weighted average amortization period of 1.5 years