Gas and Liquids Stored Underground and Gas and NGLs Receivables and Payables (Details) MMBbls in Millions, MMBTU in Millions, $ in Millions	Mar. 31, 2019 USD ($) $ / MMBTU MMBTU MMBbls	Dec. 31, 2018 USD ($) $ / MMBTU MMBTU MMBbls
Gas Balancing Arrangements [Abstract]
Gas Balancing Measurement (in MMBtu) | MMBTU	32.3	13.5
Average Market Price of Gas Assumed | $ / MMBTU	2.83	3.68
Gas Imbalance to Subsidiaries Asset Liability	$ 91.4	$ 49.7
Natural Gas Liquids Balancing Volume (in MMBbls) (Less than) | MMBbls	0.2	0.0
Natural Gas Liquids Imbalance to Subsidiaries Asset Liability	$ 3.4