SCHEDULE III REAL ESTATE AND ACCUMULATED DEPRECIATION - Changes in real estate investments and accumulated depreciation (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Real estate investments, at cost:
Balance, beginning of year	$ 2,127,907	$ 2,058,352	$ 2,008,823
Improvements	75,472	74,324	53,279
Dispositions	(62,646)	(4,769)	(3,750)
Balance, end of year	2,140,733	2,127,907	2,058,352
Accumulated depreciation:
Balance, beginning of year	490,697	432,579	376,131
Depreciation	67,001	62,887	60,198
Dispositions	(18,981)	(4,769)	(3,750)
Balance, end of year	$ 538,717	$ 490,697	$ 432,579