Real Estate Investments - Purchase Price Allocation (Details) - USD ($) $ in Thousands		Dec. 19, 2018	Dec. 11, 2018	Jan. 05, 2018	Oct. 13, 2017	Jul. 28, 2017	Nov. 01, 2018
Quarry Lake II [Member]
Business Acquisition [Line Items]
Acquisition Date		Dec. 19, 2018
Building, land and improvements		$ 35,120
Intangible assets acquired		5,809
Below market lease liabilities assumed		(1,524)
Deferred gain		0
Total unencumbered acquisition value		39,405
Mortgage debt assumed - at fair value		0
Total encumbered acquisition value		39,405
Total unencumbered acquisition value		39,405
Mortgage debt repaid at settlement (c)		0
Investment in unconsolidated real estate ventures		0
Gain on promoted interest in unconsolidated real estate venture		0
Gain on real estate venture transactions		0
Purchase price reduction for note receivable		0
Net working capital assumed		(368)
Total cash payment at settlement		$ 39,037
Quarry Lake II [Member] | Below Market Lease [Member]
Business Acquisition [Line Items]
Weighted average amortization period		3 years
DRA Austin Venture [Member]
Business Acquisition [Line Items]
Acquisition Date			Dec. 11, 2018
Building, land and improvements			$ 457,390
Intangible assets acquired			76,925
Below market lease liabilities assumed			(13,769)
Deferred gain			14,594
Total unencumbered acquisition value			535,140
Mortgage debt assumed - at fair value			0				$ (115,500)
Total encumbered acquisition value			535,140
Total unencumbered acquisition value			535,140
Mortgage debt repaid at settlement (c)			(115,461)
Investment in unconsolidated real estate ventures			(14,594)
Gain on promoted interest in unconsolidated real estate venture			(28,283)
Gain on real estate venture transactions			(103,847)
Purchase price reduction for note receivable			(130,742)
Net working capital assumed			(24,865)
Total cash payment at settlement			$ 117,348
DRA Austin Venture [Member] | Intangible Assets Acquired [Member]
Business Acquisition [Line Items]
Weighted average amortization period			5 years 6 months
DRA Austin Venture [Member] | Below Market Lease [Member]
Business Acquisition [Line Items]
Weighted average amortization period			4 years 7 months 6 days
Four Tower Bridge [Member]
Business Acquisition [Line Items]
Acquisition Date				Jan. 05, 2018
Building, land and improvements				$ 20,734
Intangible assets acquired				3,144
Below market lease liabilities assumed				(182)
Deferred gain				0
Total unencumbered acquisition value				23,696
Mortgage debt assumed - at fair value				(9,940)
Total encumbered acquisition value				13,756
Total unencumbered acquisition value				23,696
Mortgage debt repaid at settlement (c)				0
Investment in unconsolidated real estate ventures				(3,502)
Gain on promoted interest in unconsolidated real estate venture				0
Gain on real estate venture transactions				(11,633)
Purchase price reduction for note receivable				0
Net working capital assumed				1,379
Total cash payment at settlement				$ 0
Four Tower Bridge [Member] | Intangible Assets Acquired [Member]
Business Acquisition [Line Items]
Weighted average amortization period				4 years 1 month 6 days
Four Tower Bridge [Member] | Below Market Lease [Member]
Business Acquisition [Line Items]
Weighted average amortization period				4 years 9 months 18 days
Bulletin Building [Member]
Business Acquisition [Line Items]
Building, land and improvements					$ 30,583
Intangible assets acquired	[1]				10,575
Below market lease liabilities assumed	[2]				(4,055)
Total encumbered acquisition value					37,775
Construction-in-progress					$ 672
Bulletin Building [Member] | Intangible Assets Acquired [Member]
Business Acquisition [Line Items]
Weighted average amortization period					7 years 10 months 24 days
Bulletin Building [Member] | Below Market Lease [Member]
Business Acquisition [Line Items]
Weighted average amortization period					7 years
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 encumbered acquisition value						$ 32,748
3000 Market Street [Member] | Intangible Assets Acquired [Member]
Business Acquisition [Line Items]
Weighted average amortization period						5 years 10 months 24 days
3000 Market Street [Member] | Below Market Lease [Member]
Business Acquisition [Line Items]
Weighted average amortization period						6 years

[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.