Real Estate Investments, Net - Narrative (Details) € in Millions, £ in Millions	1 Months Ended	3 Months Ended	12 Months Ended
	Jan. 31, 2018 EUR (€)	Dec. 31, 2017 GBP (£)	Dec. 31, 2018 USD ($) property	Dec. 31, 2017 USD ($) property	Dec. 31, 2016 USD ($) property
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Impairment charges for the intangible assets				$ 0	$ 0
Number of properties sold (property) | property					34
(Loss) gain on dispositions of real estate investments			$ (5,751,000)	1,089,000	$ 13,341,000
Proceeds from dispositions of real estate investments			$ 23,717,000	$ 12,292,000	107,789,000
Gains on dispositions of real estate investments, before offset					14,300,000
Properties with tenant financial difficulties (property) | property			6
Impairment charges and related lease intangible write-offs			$ 1,600,000
Lease intangible write-off			$ 3,400,000
Number of real estate properties held for sale (property) | property			3	0
Advisor
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
(Loss) gain on dispositions of real estate investments				$ 800,000	$ 1,000,000
Kulicke & Soffa
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of properties sold (property) | property				1
(Loss) gain on dispositions of real estate investments				$ 400,000
Proceeds from dispositions of real estate investments				$ 12,300,000
Disposal Group, Disposed of by Sale, Not Discontinued Operations
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of properties sold (property) | property			2
Total contract price			$ 25,300,000
(Loss) gain on dispositions of real estate investments			(5,800,000)
Gain (Loss) on termination of lease			3,000,000
Mortgage notes payable
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Repayments of debt	€ 0.1	£ 0.8	$ 1,700,000