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Schedule III - REAL ESTATE ASSETS AND ACCUMULATED DEPRECIATION Activity (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Reconciliation of Carrying Amount of Real Estate Investments and Accumulated Depreciation [Roll Forward]      
Balance at beginning of period $ 7,947,647 $ 8,240,521  
Additions and improvements 177,482 263,265 $ 230,990
Acquisitions of real estate assets 78,516 0 182,747
Disposals, deconsolidations and accumulated depreciation on impairments (506,399) (435,331) (249,716)
Transfers from real estate assets (3,915) (3,986) (4,738)
Impairment of real estate assets (71,401) (116,822) (105,945)
Balance at end of period 7,621,930 7,947,647 8,240,521
Accumulated depreciation, beginning of period 2,427,108 2,382,568  
Depreciation expense 272,945 272,697 274,544
Accumulated depreciation on real estate assets sold, retired, deconsolidated or impaired (234,958) (228,157) (131,983)
Accumulated depreciation, end of period $ 2,465,095 $ 2,427,108 $ 2,382,568