Details of Selected Consolidated Balance Sheet Captions - Deferred Charges and Other Assets (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Sep. 30, 2020
Balance Sheet Related Disclosures [Abstract]
Marketable securities	$ 108,071	$ 103,952
Regulatory assets (See Note 2)	351,843	371,707
Operating lease right of use assets (See Note 6)	222,446	227,146
Winter Storm Uri incremental costs	89,009	0
Assets from risk management activities	175,613	74,991
Other	27,738	23,374
Total	974,720	$ 801,170
Gas purchases, noncurrent	76,700
Deferred non cash carrying costs, noncurrent	$ 12,300