Schedule III - Real Estate and Accumulated Depreciation (Details 2) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Real Estate And Accumulated Depreciation [Line Items]
Federal income tax purposes	$ 1,700,000
Real Estate:
Beginning balance	2,679,737	$ 2,643,065	$ 2,501,844
Additions to/improvements of real estate	63,795	57,611	153,163
Assets sold/written-off	(27,603)	(20,939)	(11,942)
Ending balance	2,715,929	2,679,737	2,643,065
Accumulated Depreciation
Beginning balance	616,175	558,657	492,582
Depreciation expense	73,659	72,191	70,592
Assets sold/written-off	(12,809)	(14,673)	(4,517)
Ending balance	$ 677,025	$ 616,175	$ 558,657
Land improvements
Real Estate And Accumulated Depreciation [Line Items]
Useful life	7 years
Building
Real Estate And Accumulated Depreciation [Line Items]
Useful life	50 years
Buildings and improvements | Minimum
Real Estate And Accumulated Depreciation [Line Items]
Useful life	7 years
Buildings and improvements | Maximum
Real Estate And Accumulated Depreciation [Line Items]
Useful life	15 years
Leasehold improvements
Real Estate And Accumulated Depreciation [Line Items]
Useful life	15 years