Gas and Liquids Stored Underground and Gas and NGLs Receivables and Payables (Details) MMBTU in Millions, $ in Millions	Sep. 30, 2015 USD ($) $ / MMBTU MMBTU MMBbls	Dec. 31, 2014 MMBTU MMBbls
Gas Balancing Arrangements [Abstract]
Gas Balancing Volume Amount (MMBtu in millions) | MMBTU	5.9	10.0
Average market price per million British thermal units | $ / MMBTU	2.53
Market value of gas owed to operating subsidiaries due to gas imbalances and gas loaned under PAL and no-notice services	$ 14.9
Natural Gas Liquids Balancing Volume (in MMBbls) (2014 - less than) | MMBbls	0.1	0.1
Natural Gas Liquids Imbalance to Subsidiaries Asset Liability	$ 2.0