Investments in Real Estate - Properties with Existing Leases (Details) $ in Thousands	3 Months Ended	6 Months Ended
	Mar. 31, 2018 USD ($) property	Jun. 30, 2018 USD ($) property	Jun. 30, 2017 USD ($) item property
Net decrease to rental revenue
2018		$ (7,856)
2019		(15,108)
2020		(14,378)
2021		(13,208)
2022		(13,074)
Thereafter		(47,962)
Totals		(111,586)
Increase to amortization expense
2018		53,176
2019		97,639
2020		91,887
2021		83,903
2022		71,978
Thereafter		407,368
Totals		805,951
In-Place Leases
Investments in real estate properties
Depreciation and amortization expense	$ 53,100		$ 51,200
Rental Revenue
Investments in real estate properties
Amortization of above and below Market Leases	7,900		6,500
Real Estate Investment | New and under development or expansion
Investments in real estate properties
Value of properties acquired during the period		$ 856,800	$ 691,900
Number of properties acquired during the period | property		358	126
Initial weighted average contractual lease rate of properties acquired (as a percent)		6.30%	6.30%
Real Estate Investment | New and under development or expansion | In-Place Leases
Investments in real estate properties
Value of properties acquired during the period	$ 225,800		$ 536,200
Number of properties acquired during the period	107		60
Real Estate Investment | Properties under development
Investments in real estate properties
Value of properties acquired during the period		$ 64,300	$ 9,900
Number of properties acquired during the period | property		10	12
Initial weighted average contractual lease rate of properties acquired (as a percent)		6.70%	7.90%