Commitments and Contingencies (Details) - USD ($) $ in Thousands	Sep. 30, 2025	Sep. 30, 2024
Recorded Unconditional Purchase Obligation [Line Items]
Estimate minimum liability for environmental remediation	$ 3,200
Other regulatory liabilities	735,938	$ 749,574
Future purchase obligation first year	236,600
Future purchase obligation second year	75,800
Future purchase obligation third year	113,800
Future purchase obligation fourth year	123,200
Future purchase obligation fifth year	135,200
Future purchase obligation thereafter	900,100
System Modernization and Expansion Projects
Recorded Unconditional Purchase Obligation [Line Items]
Contract commitments first year	58,100
Contract commitments second year	9,200
Contract commitments third year	5,600
Contract commitments fourth year	4,400
Contract commitments fifth year	4,500
Contract commitments thereafter	4,600
Well Development Activities
Recorded Unconditional Purchase Obligation [Line Items]
Contract commitments first year	180,800
Contract commitments second year	25,900
Environmental Site Remediation Costs
Recorded Unconditional Purchase Obligation [Line Items]
Other regulatory liabilities	$ 1,800