SCHEDULE III REAL ESTATE AND ACCUMULATED DEPRECIATION - Analysis of Carrying Amount of Real Estate Properties and Accumulated Depreciation (Details) - USD ($) $ in Thousands	3 Months Ended			12 Months Ended
	Mar. 31, 2018	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Real Estate Properties
Additions				$ 56,300	$ 55,000	$ 110,700
Loss on asset impairment	$ (12,100)	$ (18,400)	$ (1,300)	(12,087)	(19,714)	(58,476)
Properties classified to held for sale					(138,130)
Balance at the end of the year				1,139,642
Accumulated Depreciation
Balance at the beginning of the year	450,718		755,255	450,718	755,255	898,939
Additions				39,161	71,979	101,647
Loss on asset impairment				0	0	(119)
Properties classified to held for sale					(62,064)
Disposals				(113,911)	(314,452)	(245,212)
Balance at the end of the year				375,968	450,718	755,255
Real Estate Properties
Real Estate Properties
Balance at the beginning of the year	$ 1,747,611		$ 2,856,890	1,747,611	2,856,890	3,887,352
Additions				58,618	55,041	113,258
Loss on asset impairment				(12,032)	(6,711)	(58,595)
Disposals				(654,555)	(1,019,479)	(1,085,125)
Balance at the end of the year				$ 1,139,642	$ 1,747,611	$ 2,856,890