Schedule III - Real Estate and Accumulated Depreciation (Details 2) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Federal income tax purposes	$ 1,700,000
Real Estate:
Beginning balance	2,255,260	$ 2,214,057
Additions to/improvements of real estate	255,527	49,685	$ 191,846
Assets sold/written-off	(8,943)	(8,482)	(4,661)
Ending balance	2,501,844	2,255,260	2,214,057
Accumulated Depreciation
Beginning balance	429,814	371,563
Depreciation expense	65,723	60,856	57,746
Assets sold/written-off	(2,955)	(2,605)	(3,767)
Ending balance	$ 492,582	$ 429,814	$ 371,563
Land improvements
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Useful life	7 years
Building
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Useful life	50 years
Buildings and improvements | Minimum
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Useful life	7 years
Buildings and improvements | Maximum
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Useful life	15 years
Leasehold improvements
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Useful life	15 years