Segment Reporting Segment Reporting (Second Generation Capital Expenditures) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting Information [Line Items]
Payments For Second Generation Tenant Improvements Leasing Costs, Building Improvements and development costs	$ 52,554	$ 59,349	$ 61,900
Operating Segments [Member]
Segment Reporting Information [Line Items]
Payments For Second Generation Tenant Improvements Leasing Costs, Building Improvements and development costs	52,554	59,349	61,900
Operating Segments [Member] | Industrial [Member]
Segment Reporting Information [Line Items]
Payments For Second Generation Tenant Improvements Leasing Costs, Building Improvements and development costs	50,721	51,785	45,716
Operating Segments [Member] | Non-reportable segments
Segment Reporting Information [Line Items]
Payments For Second Generation Tenant Improvements Leasing Costs, Building Improvements and development costs	$ 1,833	$ 7,564	$ 16,184