Accounting Policies Revenue, Initial Application (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2018	Jan. 01, 2018
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Cumulative Effect of New Accounting Principle in Period of Adoption	$ (12.8)
Current Gas Stored Underground Cumulative Effect Decrease	$ 6.4
Difference between Revenue Guidance in Effect before and after Topic 606 | Accounting Standards Update 2014-09
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Cumulative Effect of New Accounting Principle in Period of Adoption		$ (12.8)
Other Liabilities Cumulative Effect Increase		6.4
Current Gas Stored Underground Cumulative Effect Decrease		$ 6.4
Minimum
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Service Contracts Duration	1 year
Expected Payment of Invoiced Services	10 days
Maximum
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Service Contracts Duration	20 years
Expected Payment of Invoiced Services	30 days