Investments - Summary of Real Estate and Loan Activity, Net of Accumulated Depreciation and Amortization (Parenthetical) (Details) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019 USD ($)	Jun. 30, 2018 USD ($)	Jun. 30, 2019 USD ($) property	Jun. 30, 2018 USD ($)	Dec. 31, 2018 USD ($)
Real Estate Properties [Line Items]
Revenue producing capitalized expenditures			$ 1,000
Non-revenue producing capitalized expenditures			2,600
Gain on disposition of real estate assets	$ 1,041	$ 4,948	1,519	$ 3,254
Dollar Amount of Investments
Held in Use land and buildings, net of accumulated depreciation	1,689,516		1,689,516		$ 1,937,867
Intangible lease assets, net	71,051		71,051		79,314
Real estate assets held for sale, net	28,090		28,090		7,263
Intangible lease liabilities, net	(15,889)		(15,889)		(17,676)
Net investments	1,815,062		$ 1,815,062		$ 2,054,537
Shopko CMBS Loans | Secured debt
Number of Properties
Dispositions of real estate (in properties) | property			83
Dollar Amount of Investments
Dispositions			$ 167,600
Held in Use
Real Estate Properties [Line Items]
Gain on disposition of real estate assets			$ 400
Number of Properties
Dispositions of real estate (in properties) | property			89
Dollar Amount of Investments
Net investments	1,744,678		$ 1,744,678
Held for Sale
Real Estate Properties [Line Items]
Gain on disposition of real estate assets			$ 1,100
Number of Properties
Dispositions of real estate (in properties) | property			6
Dollar Amount of Investments
Net investments	28,090		$ 28,090
Wholly Owned Properties
Number of Properties
Dispositions of real estate (in properties) | property			95
Dollar Amount of Investments
Held in Use land and buildings, net of accumulated depreciation	1,689,516		$ 1,689,516
Intangible lease assets, net	71,051		71,051
Real estate assets held for sale, net	28,090		28,090
Intangible lease liabilities, net	(15,889)		(15,889)
Net investments	$ 1,772,768		$ 1,772,768