Commitments and Contingencies (Details) - Mid-Tex Division [Member] $ in Millions	12 Months Ended
	Sep. 30, 2017 USD ($) MMcf	Sep. 30, 2016 USD ($)	Sep. 30, 2015 USD ($)
Long Term Purchase Commitment [Line Items]
Significant Purchase Commitment, Amount | $	$ 49.7	$ 85.3	$ 113.3
Inventories Under Indexed Contracts [Member]
Long Term Purchase Commitment [Line Items]
Long Term Purchase Commitment Minimum Quantity Required Within One Year	20,900
Long Term Purchase Commitment Minimum Quantity Required One To Three Years	37,900
Long Term Purchase Commitment Minimum Quantity Required After Three Years	300