Condensed Consolidated Statements of Income (unaudited) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2018	Mar. 31, 2017
Income Statement [Abstract]
Revenues	$ 483,261	$ 359,940	$ 806,366	$ 621,353
Costs and expenses:
Cost of sales — gas, fuel and purchased power (excluding depreciation shown below)	257,302	164,541	409,076	274,012
Operating and administrative expenses	67,054	58,021	119,038	107,737
Operating and administrative expenses — related parties	3,766	4,534	6,455	7,098
Depreciation and amortization	21,158	17,699	41,512	35,090
Other operating income, net	(1,146)	(1,263)	(1,137)	(1,228)
Total costs and expenses	348,134	243,532	574,944	422,709
Operating income	135,127	116,408	231,422	198,644
Interest expense	11,091	10,322	22,030	20,350
Income before income taxes	124,036	106,086	209,392	178,294
Income taxes	34,852	40,961	51,905	68,904
Net income	$ 89,184	$ 65,125	$ 157,487	$ 109,390