Fair Value Measures - Narrative (Details)	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
Impairment charges	$ 50,548,000	$ 182,819,000	$ 91,755,000
Percentage of fair value in excess of carrying value	8.10%	21.00%	13.00%
Goodwill impairment charges	$ 0	$ 0	$ 0
Amount of fair value in excess of carrying value	1,100,000,000
Amount of fair value in excess of carrying value, with 0.5% increase in discount rate	$ 385,000,000
Minimum [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Discount rates	7.40%
Capitalization rates	6.90%
Maximum [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Discount rates	7.80%
Capitalization rates	10.00%
Weighted Average [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Discount rates	7.50%
Capitalization rates	8.00%
Reported Value Measurement [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying value of real estate assets deemed to be impaired	$ 161,900,000	668,200,000	340,100,000
Estimate of Fair Value Measurement [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value of real estate assets deemed to be impaired	111,400,000	485,400,000	248,300,000
Carrying value	$ 15,100,000,000	$ 18,300,000,000	$ 19,700,000,000