Financial Instruments - Narrative (Details)	1 Months Ended	12 Months Ended
	Jun. 30, 2017 USD ($)	Sep. 30, 2018 USD ($) $ / Mcf Bcf MMcf	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)	Sep. 30, 2015 USD ($)
Derivative [Line Items]
Net loss on settled Treasury lock agreements reclassified from AOCI into interest expense		$ (2,400,000)	$ (1,000,000)	$ (500,000)
Interest Rate Hedges, Senior Notes 2017 Issuance
Derivative [Line Items]
Derivative, Notional Amount			210,000,000
Anticipated Debt Issuance, Amount			$ 250,000,000
Derivative, Cash Paid on Hedge	$ 37,000,000
Long-term Debt, Term			27 years
Interest Rate Hedges, Senior Notes 2019 Issuance
Derivative [Line Items]
Derivative, Notional Amount					$ 450,000,000
Anticipated Debt Issuance, Amount					$ 450,000,000
Not Designated as Hedging Instrument | Gas Purchases | Commodity contracts
Derivative [Line Items]
Hedging percent		26.00%
Purchase commitment volume | Bcf		15,000
Weighted average cost | $ / Mcf		3.20
Not Designated as Hedging Instrument | Gas Purchases | Commodity contracts | Minimum
Derivative [Line Items]
Hedging percent		25.00%
Not Designated as Hedging Instrument | Gas Purchases | Commodity contracts | Maximum
Derivative [Line Items]
Hedging percent		50.00%
Not Designated as Hedging Instrument | Gas Purchases | Commodity contracts | Long
Derivative [Line Items]
Purchase commitment volume | MMcf		22,874