Investments - Summary of Gross Investment Activity (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2019 USD ($) InvestmentLocation	Mar. 31, 2018 USD ($) InvestmentLocation	Dec. 31, 2018 USD ($)
Real Estate Investments [Abstract]
Number of Investment Locations | InvestmentLocation	677	508
Number of Investment Locations, Acquisitions of and additions to real estate investments | InvestmentLocation	51	28
Number of Investment Locations, Sales of investments in real estate | InvestmentLocation	(7)	(6)
Number of Investment Locations, Relinquishment of properties at end of ground lease term | InvestmentLocation	(1)
Number of Investment Locations, Principal collections on and settlements of loans and direct financing lease receivables | InvestmentLocation	(9)
Number of Investment Locations | InvestmentLocation	711	530
Gross investments	$ 1,394,549	$ 939,072
Acquisitions of and additions to real estate investments	123,896	64,099
Sales of investments in real estate	(10,651)	(7,152)
Relinquishment of properties at end of ground lease term	(241)
Provisions for impairment of real estate	(1,440)	(1,861)
Investments in loans receivable	207
Principal collections on loans and direct financing lease receivables		(18)
Principal collections on and settlements of loans and direct financing lease receivables	(5,769)
Other	(501)
Gross investments	1,500,050	994,140
Less: accumulated depreciation and amortization	(60,230)	(31,310)	$ (51,855)
Net investments	$ 1,439,820	$ 962,830	$ 1,342,694