Schedule III - Real Estate and Accumulated Depreciation (Details 2) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Federal income tax purposes	$ 1,600,000
Real Estate:
Beginning balance	2,026,872	$ 1,935,556	$ 1,840,306
Additions to/improvements of real estate	191,846	94,521	97,302
Assets sold/written-off	(4,661)	(3,205)	(2,052)
Ending balance	2,214,057	2,026,872	1,935,556
Accumulated Depreciation
Beginning balance	317,584	266,848	217,428
Depreciation expense	57,746	53,182	50,616
Assets sold/written-off	(3,767)	(2,446)	(1,196)
Ending balance	$ 371,563	$ 317,584	$ 266,848
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