Fair Value Measures - Impairment Charges by Asset Class (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2018 USD ($) property	Mar. 31, 2017 USD ($) property
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Properties impaired | property	12	14
Total impairment loss	$ 6,036	$ 6,725
Investment in Real Estate Assets, Net [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total impairment loss	6,043	6,748
Below-Market Lease Liabilities, Net [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total impairment loss	$ (7)	$ (23)