Derivative Instruments and Hedging Activities - Exchange-Traded/Cleared Derivatives (Details) MMBTU in Thousands	12 Months Ended
	Sep. 30, 2019 MMBTU $ / MMBTU
Short | Natural Gas Contracts | Gas Utility | Fiscal 2020
Derivative [Line Items]
MMBtu (millions) | MMBTU	0
Avg. Price Per MMBtu | $ / MMBTU	0
Short | Natural Gas Contracts | Gas Utility | Fiscal 2021
Derivative [Line Items]
MMBtu (millions) | MMBTU	0
Avg. Price Per MMBtu | $ / MMBTU	0
Short | Natural Gas Contracts | Gas Marketing | Fiscal 2020
Derivative [Line Items]
MMBtu (millions) | MMBTU	10,040
Avg. Price Per MMBtu | $ / MMBTU	2.77
Short | Natural Gas Contracts | Gas Marketing | Fiscal 2021
Derivative [Line Items]
MMBtu (millions) | MMBTU	1,760
Avg. Price Per MMBtu | $ / MMBTU	2.71
Short | ICE Open Basis Swap Positions | Gas Utility | Fiscal 2020
Derivative [Line Items]
MMBtu (millions) | MMBTU	0
Avg. Price Per MMBtu | $ / MMBTU	0
Short | ICE Open Basis Swap Positions | Gas Utility | Fiscal 2021
Derivative [Line Items]
MMBtu (millions) | MMBTU	0
Avg. Price Per MMBtu | $ / MMBTU	0
Short | ICE Open Basis Swap Positions | Gas Utility | Fiscal 2022
Derivative [Line Items]
MMBtu (millions) | MMBTU	0
Avg. Price Per MMBtu | $ / MMBTU	0
Short | ICE Open Basis Swap Positions | Gas Marketing | Fiscal 2020
Derivative [Line Items]
MMBtu (millions) | MMBTU	53,060
Avg. Price Per MMBtu | $ / MMBTU	0.13
Short | ICE Open Basis Swap Positions | Gas Marketing | Fiscal 2021
Derivative [Line Items]
MMBtu (millions) | MMBTU	5,350
Avg. Price Per MMBtu | $ / MMBTU	0.25
Short | ICE Open Basis Swap Positions | Gas Marketing | Fiscal 2022
Derivative [Line Items]
MMBtu (millions) | MMBTU	310
Avg. Price Per MMBtu | $ / MMBTU	0.25
Short | ICE Open Daily Swap Positions | Gas Utility | Fiscal 2020
Derivative [Line Items]
MMBtu (millions) | MMBTU	0
Avg. Price Per MMBtu | $ / MMBTU	0
Short | ICE Open Daily Swap Positions | Gas Marketing | Fiscal 2020
Derivative [Line Items]
MMBtu (millions) | MMBTU	1,120
Avg. Price Per MMBtu | $ / MMBTU	2.00
Short | ICE Puts Positions | Gas Marketing | Fiscal 2020
Derivative [Line Items]
MMBtu (millions) | MMBTU	4,800
Avg. Price Per MMBtu | $ / MMBTU	0.14
Long | Natural Gas Contracts | Gas Utility | Fiscal 2020
Derivative [Line Items]
MMBtu (millions) | MMBTU	66,360
Avg. Price Per MMBtu | $ / MMBTU	2.66
Long | Natural Gas Contracts | Gas Utility | Fiscal 2021
Derivative [Line Items]
MMBtu (millions) | MMBTU	2,680
Avg. Price Per MMBtu | $ / MMBTU	2.68
Long | Natural Gas Contracts | Gas Utility | Fiscal 2022
Derivative [Line Items]
MMBtu (millions) | MMBTU	0
Avg. Price Per MMBtu | $ / MMBTU	0
Long | Natural Gas Contracts | Gas Utility | Fiscal 2023
Derivative [Line Items]
MMBtu (millions) | MMBTU	0
Avg. Price Per MMBtu | $ / MMBTU	0
Long | Natural Gas Contracts | Gas Marketing | Fiscal 2020
Derivative [Line Items]
MMBtu (millions) | MMBTU	11,000
Avg. Price Per MMBtu | $ / MMBTU	2.63
Long | Natural Gas Contracts | Gas Marketing | Fiscal 2021
Derivative [Line Items]
MMBtu (millions) | MMBTU	4,910
Avg. Price Per MMBtu | $ / MMBTU	2.64
Long | Natural Gas Contracts | Gas Marketing | Fiscal 2022
Derivative [Line Items]
MMBtu (millions) | MMBTU	1,130
Avg. Price Per MMBtu | $ / MMBTU	2.73
Long | Natural Gas Contracts | Gas Marketing | Fiscal 2023
Derivative [Line Items]
MMBtu (millions) | MMBTU	220
Avg. Price Per MMBtu | $ / MMBTU	2.69
Long | ICE Open Basis Swap Positions | Gas Utility | Fiscal 2020
Derivative [Line Items]
MMBtu (millions) | MMBTU	0
Avg. Price Per MMBtu | $ / MMBTU	0
Long | ICE Open Basis Swap Positions | Gas Utility | Fiscal 2021
Derivative [Line Items]
MMBtu (millions) | MMBTU	0
Avg. Price Per MMBtu | $ / MMBTU	0
Long | ICE Open Basis Swap Positions | Gas Utility | Fiscal 2022
Derivative [Line Items]
MMBtu (millions) | MMBTU	0
Avg. Price Per MMBtu | $ / MMBTU	0
Long | ICE Open Basis Swap Positions | Gas Marketing | Fiscal 2020
Derivative [Line Items]
MMBtu (millions) | MMBTU	28,950
Avg. Price Per MMBtu | $ / MMBTU	0.45
Long | ICE Open Basis Swap Positions | Gas Marketing | Fiscal 2021
Derivative [Line Items]
MMBtu (millions) | MMBTU	1,320
Avg. Price Per MMBtu | $ / MMBTU	0.49
Long | ICE Open Basis Swap Positions | Gas Marketing | Fiscal 2022
Derivative [Line Items]
MMBtu (millions) | MMBTU	920
Avg. Price Per MMBtu | $ / MMBTU	0.52
Long | ICE Open Basis Swap Positions | Gas Marketing | Fiscal 2023
Derivative [Line Items]
MMBtu (millions) | MMBTU	160
Avg. Price Per MMBtu | $ / MMBTU	0.52
Long | ICE Open Daily Swap Positions | Gas Utility | Fiscal 2020
Derivative [Line Items]
MMBtu (millions) | MMBTU	0
Avg. Price Per MMBtu | $ / MMBTU	0
Long | ICE Open Daily Swap Positions | Gas Marketing | Fiscal 2020
Derivative [Line Items]
MMBtu (millions) | MMBTU	270
Avg. Price Per MMBtu | $ / MMBTU	2.38
Long | ICE Calls Positions | Gas Utility | Fiscal 2020
Derivative [Line Items]
MMBtu (millions) | MMBTU	0
Avg. Price Per MMBtu | $ / MMBTU	0
Long | ICE Calls Positions | Gas Marketing | Fiscal 2020
Derivative [Line Items]
MMBtu (millions) | MMBTU	2,100
Avg. Price Per MMBtu | $ / MMBTU	0.10