Investments in Real Estate - Schedule of Acquisitions (Details) £ in Millions, ft² in Millions, $ in Millions	9 Months Ended
	Sep. 30, 2019 GBP (£) ft² state property	Sep. 30, 2019 USD ($) ft² state property	Sep. 30, 2018 USD ($) ft² property
Real Estate [Line Items]
Number of properties | property	241	241	591
Square feet (sq ft) | ft²	7.8	7.8	4.3
Investment | $		$ 2,025.7	$ 1,465.4
Weighted average lease term (years)	15 years 6 months	15 years 6 months	14 years 4 months 24 days
Initial average cash lease yield (percent)	6.20%	6.20%	6.30%
New properties
Real Estate [Line Items]
Number of properties, new | property	227	227
Square feet (sq ft) | ft²	7.4	7.4
Investment | $		$ 1,989.7
Weighted average lease term (years)	15 years 6 months	15 years 6 months
Initial average cash lease yield (percent)	6.10%	6.10%
New properties | U.S.
Real Estate [Line Items]
Number of properties, new | property	214	214	580
Number of states	38	38	37
Square feet (sq ft) | ft²	6.2	6.2	3.2
Investment | $		$ 1,412.9	$ 1,395.7
Weighted average lease term (years)	15 years 8 months 12 days	15 years 8 months 12 days	14 years 7 months 6 days
Initial average cash lease yield (percent)	6.50%	6.50%	6.30%
New properties | U.K.
Real Estate [Line Items]
Number of properties, new | property	13	13
Square feet (sq ft) | ft²	1.2	1.2
Investment	£ 456.1	$ 576.8
Weighted average lease term (years)	15 years	15 years
Initial average cash lease yield (percent)	5.20%	5.20%
Properties under development | U.S.
Real Estate [Line Items]
Number of properties, under development | property	14	14	11
Square feet (sq ft) | ft²	0.4	0.4	1.1
Investment | $		$ 36.0	$ 69.7
Weighted average lease term (years)	16 years	16 years	10 years 10 months 24 days
Initial average cash lease yield (percent)	7.40%	7.40%	6.80%