Real Estate Investments - Purchase Price Allocation (Parenthetical) (Details)	Oct. 13, 2017	Jul. 28, 2017	Jun. 22, 2015
One Drexel Plaza [Member] | Intangible Assets Acquired [Member]
Business Acquisition [Line Items]
Weighted average amortization period	7 years 10 months 24 days
One Drexel Plaza [Member] | Below Market Lease [Member]
Business Acquisition [Line Items]
Weighted average amortization period	7 years
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
Broadmoor Austin Associates [Member] | Intangible Assets Acquired [Member]
Business Acquisition [Line Items]
Weighted average amortization period			4 years
Broadmoor Austin Associates [Member] | Below Market Lease [Member]
Business Acquisition [Line Items]
Weighted average amortization period			1 year 6 months