Fair Value Measures - Impairment Charges by Asset Class (Details) $ in Thousands	9 Months Ended
	Sep. 30, 2018 USD ($) property	Sep. 30, 2017 USD ($) property
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Properties impaired | property	53	53
Impairment of below market leases	$ (222)	$ (23)
Total impairment loss	36,082	30,857
Real Estate Assets [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total impairment loss	35,196	30,327
Direct Financing Leases [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total impairment loss	$ 1,108	$ 553