Investments in Real Estate - Schedule of Acquisitions (Details) £ in Millions, ft² in Millions, $ in Millions	3 Months Ended			6 Months Ended
	Jun. 30, 2019 GBP (£) ft² property	Jun. 30, 2019 USD ($) ft² property	Jun. 30, 2018 USD ($) ft² property	Jun. 30, 2019 USD ($) ft² property	Jun. 30, 2018 USD ($) ft² property
Real Estate [Line Items]
Number of properties | property	102	102	190	199	358
Square feet (sq ft) | ft²	3.8	3.8	1.9	5.7	2.8
Investment | $		$ 1,094.7	$ 347.0	$ 1,614.2	$ 856.8
Weighted average lease term (years)	14 years 9 months 18 days	14 years 9 months 18 days	13 years 7 months 6 days	15 years 7 months 6 days	13 years 9 months 18 days
Initial average cash lease yield (percent)	6.10%	6.10%	6.50%	6.30%	6.30%
New properties
Real Estate [Line Items]
Number of properties, new | property	90	90		187
Square feet (sq ft) | ft²	3.4	3.4		5.3
Investment | $		$ 1,081.5		$ 1,590.1
Weighted average lease term (years)	14 years 9 months 18 days	14 years 9 months 18 days		15 years 6 months
Initial average cash lease yield (percent)	6.10%	6.10%		6.30%
New properties | U.S.
Real Estate [Line Items]
Number of properties, new | property	78	78	180	175	348
Square feet (sq ft) | ft²	2.3	2.3	1.0	4.2	1.8
Investment | $		$ 532.3	$ 286.5	$ 1,040.9	$ 792.5
Weighted average lease term (years)	14 years 9 months 18 days	14 years 9 months 18 days	14 years 2 months 12 days	15 years 10 months 24 days	14 years 1 month 6 days
Initial average cash lease yield (percent)	6.90%	6.90%	6.50%	6.80%	6.30%
New properties | U.K.
Real Estate [Line Items]
Number of properties, new | property	12	12		12
Square feet (sq ft) | ft²	1.1	1.1		1.1
Investment	£ 433.9	$ 549.2		$ 549.2
Weighted average lease term (years)	14 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%
Properties under development | U.S.
Real Estate [Line Items]
Number of properties, under development | property	12	12	10	12	10
Square feet (sq ft) | ft²	0.4	0.4	0.9	0.4	1.0
Investment | $		$ 13.2	$ 60.5	$ 24.1	$ 64.3
Weighted average lease term (years)	15 years 10 months 24 days	15 years 10 months 24 days	10 years 8 months 12 days	16 years 6 months	10 years 9 months 18 days
Initial average cash lease yield (percent)	7.30%	7.30%	6.70%	7.20%	6.70%