Details of Selected Consolidated Balance Sheet Captions - Other Current Liabilities Detail Table (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Sep. 30, 2017
Offsetting [Abstract]
Customer credit balances and deposits	$ 52,648	$ 54,627
Accrued employee costs	52,101	46,653
Deferred gas costs	94,705	15,559
Accrued interest	39,486	39,624
Liabilities from risk management activities	56,734	322
Taxes payable	123,457	116,291
Pension and postretirement obligations	10,475	18,411
Regulatory cost of service reserve	22,508	0
Regulatory cost of removal obligation	55,770	35,910
APT annual adjustment mechanism, current	19,918	0
Regulatory excess deferred taxes (See Note 12)	5,225	0
Other	14,041	5,251
Total	$ 547,068	$ 332,648