Impairment Charges (Details) $ in Thousands	6 Months Ended	12 Months Ended
	Feb. 28, 2019 USD ($)	Dec. 31, 2018 USD ($) properties	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Feb. 07, 2019 properties
Impaired Long-Lived Assets Held and Used [Line Items]
Rental properties, net of accumulated depreciation of $883,174 and $741,334 at December 31, 2018 and 2017, respectively		$ 5,024,057	$ 4,604,231
Tenant Reimbursements		15,400	15,600	$ 15,600
Impairment charges		$ 27,283	10,195	$ 0
Children's Learning Adventure USA, LLC [Member]
Impaired Long-Lived Assets Held and Used [Line Items]
Property Subject to or Available for Operating Lease, Number of Units | properties		21
Payments for Rent		$ 4,300
Impaired Assets to be Disposed of by Method Other than Sale, Carrying Value of Asset		9,800
Impairment charges		$ 16,500
land held for development [Member] | Children's Learning Adventure USA, LLC [Member]
Impaired Long-Lived Assets Held and Used [Line Items]
number of properties in land held for development | properties		4
Imagine Schools [Member]
Impaired Long-Lived Assets Held and Used [Line Items]
Impairment charges			9,600
Tangible Asset Impairment Charges			$ 2,300
Tenants [Member] | early childhood education center [Member]
Impaired Long-Lived Assets Held and Used [Line Items]
Rental properties, net of accumulated depreciation of $883,174 and $741,334 at December 31, 2018 and 2017, respectively		$ 246,200
Subsequent Event [Member] | Children's Learning Adventure USA, LLC [Member]
Impaired Long-Lived Assets Held and Used [Line Items]
Payments for Rent	$ 1,000
Tenant Reimbursements	$ 170
Subsequent Event [Member] | land held for development [Member] | Children's Learning Adventure USA, LLC [Member]
Impaired Long-Lived Assets Held and Used [Line Items]
number of properties in land held for development | properties					2
Louisiana Theatre Properties [Member]
Impaired Long-Lived Assets Held and Used [Line Items]
Impairment charges		10,700
Tangible Asset Impairment Charges		7,800
Loss Contingency Accrual		$ 2,900