Fair Value Measures - Impairment Charges by Asset Class (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) property	Dec. 31, 2016 USD ($) property	Dec. 31, 2015 USD ($) property
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Properties impaired | property	69	153	202
Total impairment loss	$ 50,548	$ 182,819	$ 91,755
Investment in Real Estate Assets, Net [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total impairment loss	50,087	183,240	88,465
Investment in Direct Financing Leases, Net [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total impairment loss	553	0	4,020
Below-Market Lease Liabilities, Net [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total impairment loss	$ (92)	$ (421)	$ (730)