Gas and Liquids Stored Underground and Gas and NGLs Receivables and Payables (Details) MMBbls in Millions, MMBTU in Millions, $ in Millions	Sep. 30, 2017 USD ($) $ / MMBTU MMBTU MMBbls	Dec. 31, 2016 MMBTU MMBbls
Gas Balancing Arrangements [Abstract]
Gas Balancing Volume Amount (in MMBtu) | MMBTU	8.8	13.6
Average Market Price of Gas Assumed | $ / MMBTU	2.86
Gas Imbalance to Subsidiaries Asset Liability | $	$ 25.1
Natural Gas Liquids Balancing Volume (in MMBbls) (Less than) | MMBbls	0.0	0.1