Condensed Consolidated Statements of Income (unaudited) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2019	Mar. 31, 2018
Income Statement [Abstract]
Revenues	$ 429,592	$ 483,261	$ 752,317	$ 806,366
Costs and expenses:
Cost of sales — gas and purchased power (excluding depreciation shown below)	217,976	257,302	377,495	409,076
Operating and administrative expenses	63,932	66,410	122,872	117,819
Operating and administrative expenses — related parties	5,198	3,766	8,710	6,455
Depreciation	22,341	21,158	44,815	41,512
Other operating expense (income), net	279	(1,146)	1,485	(1,137)
Total costs and expenses	309,726	347,490	555,377	573,725
Operating income	119,866	135,771	196,940	232,641
Pension and other postretirement plans non-service income	395	(644)	807	(1,219)
Interest expense	(12,231)	(11,091)	(23,969)	(22,030)
Income before income taxes	108,030	124,036	173,778	209,392
Income tax expense	(25,170)	(34,852)	(41,030)	(51,905)
Net income	$ 82,860	$ 89,184	$ 132,748	$ 157,487