Schedule III - Real Estate and Accumulated Depreciation (Details 2) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Federal income tax purposes	$ 1,500,000
Real Estate:
Beginning balance	2,214,057	$ 2,026,872	$ 1,935,556
Additions to/improvements of real estate	49,685	191,846	94,521
Assets sold/written-off	(8,482)	(4,661)	(3,205)
Ending balance	2,255,260	2,214,057	2,026,872
Accumulated Depreciation
Beginning balance	371,563	317,584	266,848
Depreciation expense	60,856	57,746	53,182
Assets sold/written-off	(2,605)	(3,767)	(2,446)
Ending balance	$ 429,814	$ 371,563	$ 317,584
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