Dispositions (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) property	Dec. 31, 2016 USD ($) property	Dec. 31, 2015 USD ($) property
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Document Fiscal Year Focus	2017
Proceeds from sale of real estate investments		$ 137,479	$ 108,822
Gain (Loss) on Sale of Properties, Net of Applicable Income Taxes	$ 27,432	47,321	35,606
Provision for impairment	$ 0	$ 1,700	$ 0
Operating Segments [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of Real Estate Properties Sold | property	6	11	5
Land [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of Real Estate Properties Sold | property	9	16	2