Details of Selected Consolidated Balance Sheet Captions - Other Current Liabilities (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Sep. 30, 2020
Balance Sheet Related Disclosures [Abstract]
Customer credit balances and deposits	$ 49,722	$ 56,485
Accrued employee costs	50,517	48,057
Deferred gas costs	52,553	19,985
Operating leases	37,688	35,716
Accrued interest	55,164	53,554
Liabilities from risk management activities	5,269	2,015
Taxes payable	160,986	148,292
Pension and postretirement liabilities	4,863	29,609
Regulatory cost of removal obligation	72,823	73,908
APT annual adjustment mechanism	22,694	43,893
Regulatory excess deferred taxes (See Note 14)	155,857	20,887
Other	18,545	14,060
Total	$ 686,681	$ 546,461