Contingent Liabilities (Details) $ in Millions	Mar. 31, 2018 USD ($)
Contingent Liabilities [Line Items]
Accrued environmental loss liabilities	$ 17
Gas Pipeline [Member]
Contingent Liabilities [Line Items]
Accrued environmental loss liabilities	7
Natural Gas Underground Storage Facilities [Member]
Contingent Liabilities [Line Items]
Accrued environmental loss liabilities	$ 10