Investments in Real Estate - Schedule of Acquisitions (Details) £ in Millions, ft² in Millions, $ in Millions	9 Months Ended
	Sep. 30, 2020 USD ($) ft² property state	Sep. 30, 2020 GBP (£) ft² property state	Sep. 30, 2019 USD ($) ft² property	Sep. 30, 2019 GBP (£) ft² property
Real Estate [Line Items]
Number of properties | property	180	180	241	241
Leasable square feet (sq ft) | ft²	5.1	5.1	7.8	7.8
Investment | $	$ 1,298.9		$ 2,025.7
Weighted average lease term (years)	13 years 1 month 6 days	13 years 1 month 6 days	15 years 6 months	15 years 6 months
Initial average cash lease yield (percent)	6.30%	6.30%	6.20%	6.20%
New properties
Real Estate [Line Items]
Number of properties, new | property	167	167	227	227
Leasable square feet (sq ft) | ft²	4.2	4.2	7.4	7.4
Investment | $	$ 1,275.6		$ 1,989.7
Weighted average lease term (years)	13 years 1 month 6 days	13 years 1 month 6 days	15 years 6 months	15 years 6 months
Initial average cash lease yield (percent)	6.30%	6.30%	6.10%	6.10%
New properties | U.S.
Real Estate [Line Items]
Number of properties, new | property	154	154	214	214
Number of states	28	28	38	38
Leasable square feet (sq ft) | ft²	3.0	3.0	6.2	6.2
Investment | $	$ 821.9		$ 1,412.9
Weighted average lease term (years)	14 years 9 months 18 days	14 years 9 months 18 days	15 years 8 months 12 days	15 years 8 months 12 days
Initial average cash lease yield (percent)	6.20%	6.20%	6.50%	6.50%
New properties | U.K.
Real Estate [Line Items]
Number of properties, new | property	13	13	13	13
Leasable square feet (sq ft) | ft²	1.2	1.2	1.2	1.2
Investment	$ 453.7	£ 356.7	$ 576.8	£ 456.1
Weighted average lease term (years)	10 years	10 years	15 years	15 years
Initial average cash lease yield (percent)	6.40%	6.40%	5.20%	5.20%
Properties under development | U.S.
Real Estate [Line Items]
Number of properties, under development | property	13	13	14	14
Leasable square feet (sq ft) | ft²	0.9	0.9	0.4	0.4
Investment | $	$ 23.3		$ 36.0
Weighted average lease term (years)	16 years 3 months 18 days	16 years 3 months 18 days	16 years	16 years
Initial average cash lease yield (percent)	6.40%	6.40%	7.40%	7.40%