Significant Accounting Policies Insurance (Details) $ in Millions	Dec. 31, 2017 USD ($)
Liability Policies [Member]
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Self-insured retention, maximum per incident	$ 2.5
Property policies [Member]
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Self-insured retention, maximum per incident	$ 2.5