SCHEDULE III REAL ESTATE AND ACCUMULATED DEPRECIATION - Analysis of Carrying Amount of Real Estate Properties and Accumulated Depreciation (Details) - USD ($)	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Real Estate Properties
Additions		$ 11,600,000	$ 13,900,000	$ 56,300,000
Loss on asset impairment	$ (12,100,000)	0	0	(12,087,000)
Balance at the end of the year		401,710,000
Accumulated Depreciation
Balance at the beginning of the year	450,718,000	202,700,000	375,968,000	450,718,000
Additions		15,275,000	22,697,000	39,161,000
Loss on asset impairment				0
Disposals		(74,656,000)	(195,965,000)	(113,911,000)
Balance at the end of the year		143,319,000	202,700,000	375,968,000
Aggregate cost of properties for federal income tax purposes		393,044,000
Real Estate Properties
Real Estate Properties
Balance at the beginning of the year	$ 1,747,611,000	662,121,000	1,139,642,000	1,747,611,000
Additions		11,642,000	13,895,000	58,618,000
Loss on asset impairment				(12,032,000)
Disposals		(272,053,000)	(491,416,000)	(654,555,000)
Balance at the end of the year		$ 401,710,000	$ 662,121,000	$ 1,139,642,000
Buildings and improvements | Maximum
Accumulated Depreciation
Estimated useful lives		40 years
Personal property | Maximum
Accumulated Depreciation
Estimated useful lives		12 years