Investments - Summary of Gross Investment Activity (Details) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018 USD ($) InvestmentLocation	Jun. 30, 2017 USD ($) InvestmentLocation	Jun. 30, 2018 USD ($) InvestmentLocation	Jun. 30, 2017 USD ($) InvestmentLocation	Dec. 31, 2017 USD ($) InvestmentLocation	Dec. 31, 2016 InvestmentLocation
Real Estate Investments [Abstract]
Number of Investment Locations | InvestmentLocation	602	395	602	395	508	344
Number of Investment Locations, Acquisitions of and additions to real estate investments | InvestmentLocation			112	72
Number of Investment Locations, Sales of investments in real estate and direct financing lease receivables | InvestmentLocation				(21)
Number of Investment Locations, Sales of investments in real estate | InvestmentLocation			(16)
Number of Investment Locations, Relinquishment of properties at end of ground lease term | InvestmentLocation			(2)
Gross investments			$ 939,072	$ 458,667
Acquisitions of and additions to real estate investments			276,315	235,769
Sales of investments in real estate and direct financing lease receivables				(15,923)
Sales of investments in real estate			(19,301)
Relinquishment of properties at end of ground lease term			(853)
Provisions for impairment of real estate	$ (900)	$ (400)	(2,781)	(579)
Investments in loans receivable			3,632
Principal collections on direct financing lease receivables			(35)	(44)
Other			(1,491)
Gross investments	1,194,558	677,890	1,194,558	677,890
Less: accumulated depreciation and amortization	(36,310)	(14,855)	(36,310)	(14,855)	$ (24,825)
Net investments	$ 1,158,248	$ 663,035	$ 1,158,248	$ 663,035	$ 914,247