Investments in Real Estate - Acquisitions Allocation (Details) $ in Thousands, £ in Millions	9 Months Ended
	Sep. 30, 2020 USD ($)	Sep. 30, 2020 GBP (£)	Sep. 30, 2019 USD ($)	Sep. 30, 2019 GBP (£)
Real Estate Properties Acquired [Abstract]
Weighted average amortization period for acquired lease intangible assets	17 years 6 months	17 years 6 months	15 years 1 month 6 days	15 years 1 month 6 days
Weighted average amortization period for acquired lease intangible liabilities	13 years 10 months 24 days	13 years 10 months 24 days	19 years 6 months	19 years 6 months
Real Estate Investment | U.S.
Real Estate Properties Acquired [Abstract]
Land	$ 226,600		$ 280,100
Buildings and improvements	452,800		993,000
Lease intangible assets	141,800		124,600
Other assets	19,500		54,700
Lease intangible liabilities	(5,100)		(28,600)
Other liabilities	(900)		(8,400)
Net	834,700		$ 1,415,400
Real Estate Investment | U.S. | Other assets
Real Estate Properties Acquired [Abstract]
Allocated to financing receivables	18,800
Allocated to right-of-use assets	$ 689
Real Estate Investment | U.K.
Real Estate Properties Acquired [Abstract]
Land | £		£ 81.0		£ 171.3
Buildings and improvements | £		125.4		189.3
Lease intangible assets | £		86.3		98.9
Other assets | £		64.0		0.0
Lease intangible liabilities | £		0.0		(3.4)
Other liabilities | £		0.0		0.0
Net | £		356.7		456.1
Real Estate Investment | U.K. | Land
Real Estate Properties Acquired [Abstract]
Allocated to right-of-use assets | £		£ 6.4		£ 13.6