Schedule III - Real Estate and Accumulated Depreciation (Details 3) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
SEC Schedule III, Real Estate and Accumulated Depreciation Disclosure [Abstract]
Accumulated depreciation, beginning of year	$ 1,139,853	$ 994,241	$ 947,664
Depreciation of real estate	190,515	171,983	159,524
SEC Schedule III, Real Estate Accumulated Depreciation, Period Increase	190,515	171,983	159,524
Write-offs due to sale	(66,206)	(22,471)	(66,603)
Properties held for sale	0	(3,900)	(46,191)
Other deductions	0	0	(153)
Total deductions during period	(66,206)	(26,371)	(112,947)
Accumulated depreciation, end of year	$ 1,264,162	$ 1,139,853	$ 994,241