Investments in Real Estate - Acquisitions Allocation (Details) - Real Estate Investment $ in Thousands, £ in Millions	6 Months Ended
	Jun. 30, 2020 USD ($)	Jun. 30, 2020 GBP (£)	Jun. 30, 2019 USD ($)	Jun. 30, 2019 GBP (£)
U.S.
Real Estate Properties Acquired [Abstract]
Land	$ 82,900		$ 234,700
Buildings and improvements	278,400		704,500
Lease intangible assets	54,500		80,300
Other assets	1,500		53,800
Lease intangible liabilities	(2,300)		(23,000)
Other liabilities	(900)		(7,400)
Net	414,100		$ 1,042,900
U.S. | Other assets
Real Estate Properties Acquired [Abstract]
Allocated to financing receivables	810
Allocated to right-of-use assets	$ 689
U.K.
Real Estate Properties Acquired [Abstract]
Land | £		£ 22.8		£ 164.4
Buildings and improvements | £		60.8		182.0
Lease intangible assets | £		42.0		90.8
Other assets | £		54.5		0.0
Lease intangible liabilities | £		0.0		(3.3)
Other liabilities | £		0.0		0.0
Net | £		180.1		433.9
U.K. | Land
Real Estate Properties Acquired [Abstract]
Allocated to right-of-use assets | £		£ 6.4		£ 13.6