Investments in Real Estate - Acquisitions Allocation (Details) $ in Thousands, £ in Millions	12 Months Ended
	Dec. 31, 2020 USD ($)	Dec. 31, 2020 GBP (£)	Dec. 31, 2019 USD ($)	Dec. 31, 2019 GBP (£)
Real Estate Properties Acquired [Abstract]
Weighted average amortization period for acquired lease intangible assets	15 years 10 months 24 days	15 years 10 months 24 days	13 years 3 months 18 days	13 years 3 months 18 days
Weighted average amortization period for acquired lease intangible liabilities	15 years 10 months 24 days	15 years 10 months 24 days	18 years 1 month 6 days	18 years 1 month 6 days
Real Estate Investment | U.S.
Real Estate Properties Acquired [Abstract]
Land	$ 337,500		$ 780,000
Buildings and improvements	768,400		1,776,100
Lease intangible assets	203,100		290,100
Other assets	52,400		82,000
Lease intangible liabilities	(12,900)		(41,900)
Other liabilities	(900)		(8,400)
Net	1,347,600		$ 2,877,900
Real Estate Investment | U.S. | Other assets
Real Estate Properties Acquired [Abstract]
Allocated to right-of-use assets	689
Allocated to financing receivables	$ 51,700
Real Estate Investment | U.K.
Real Estate Properties Acquired [Abstract]
Land | £		£ 247.1		£ 251.0
Buildings and improvements | £		258.7		249.3
Lease intangible assets | £		139.8		129.9
Other assets | £		62.9		0.0
Lease intangible liabilities | £		(0.7)		(4.4)
Other liabilities | £		0.0		0.0
Net | £		707.8		625.8
Real Estate Investment | U.K. | Land
Real Estate Properties Acquired [Abstract]
Allocated to right-of-use assets | £		£ 88.6		£ 24.9