Investments in Real Estate - Schedule of Acquisitions (Details) £ in Millions, ft² in Millions, $ in Millions	6 Months Ended
	Jun. 30, 2020 USD ($) ft² property state	Jun. 30, 2020 GBP (£) ft² property state	Jun. 30, 2019 USD ($) ft² property	Jun. 30, 2019 GBP (£) ft² property
Real Estate [Line Items]
Number of properties | property	94	94	199	199
Square feet (sq ft) | ft²	2.5	2.5	5.7	5.7
Investment | $	$ 640.2		$ 1,614.2
Weighted average lease term (years)	13 years 7 months 6 days	13 years 7 months 6 days	15 years 7 months 6 days	15 years 7 months 6 days
Initial average cash lease yield (percent)	6.10%	6.10%	6.30%	6.30%
New properties
Real Estate [Line Items]
Number of properties, new | property	86	86	187	187
Square feet (sq ft) | ft²	2.3	2.3	5.3	5.3
Investment | $	$ 636.3		$ 1,590.1
Weighted average lease term (years)	13 years 7 months 6 days	13 years 7 months 6 days	15 years 6 months	15 years 6 months
Initial average cash lease yield (percent)	6.10%	6.10%	6.30%	6.30%
New properties | U.S.
Real Estate [Line Items]
Number of properties, new | property	80	80	175	175
Number of states	25	25	34	34
Square feet (sq ft) | ft²	1.8	1.8	4.2	4.2
Investment | $	$ 412.6		$ 1,040.9
Weighted average lease term (years)	14 years 4 months 24 days	14 years 4 months 24 days	15 years 10 months 24 days	15 years 10 months 24 days
Initial average cash lease yield (percent)	6.50%	6.50%	6.80%	6.80%
New properties | U.K.
Real Estate [Line Items]
Number of properties, new | property	6	6	12	12
Square feet (sq ft) | ft²	0.5	0.5	1.1	1.1
Investment	$ 223.7	£ 180.1	$ 549.2	£ 433.9
Weighted average lease term (years)	11 years 9 months 18 days	11 years 9 months 18 days	14 years 9 months 18 days	14 years 9 months 18 days
Initial average cash lease yield (percent)	5.30%	5.30%	5.30%	5.30%
Properties under development | U.S.
Real Estate [Line Items]
Number of properties, under development | property	8	8	12	12
Square feet (sq ft) | ft²	0.2	0.2	0.4	0.4
Investment | $	$ 3.9		$ 24.1
Weighted average lease term (years)	10 years 6 months	10 years 6 months	16 years 6 months	16 years 6 months
Initial average cash lease yield (percent)	8.80%	8.80%	7.20%	7.20%