Condensed Consolidated Statements of Income (unaudited) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Mar. 31, 2017	Mar. 31, 2016
Income Statement [Abstract]
Revenues	$ 359,940	$ 322,047	$ 621,353	$ 520,029
Costs and expenses:
Cost of sales — gas, fuel and purchased power (excluding depreciation shown below)	164,541	137,434	274,012	212,873
Operating and administrative expenses	53,064	45,125	99,101	93,152
Operating and administrative expenses — related parties	4,534	3,798	7,098	5,978
Taxes other than income taxes	4,957	4,448	8,636	8,217
Depreciation	17,125	16,146	33,987	31,973
Amortization	574	884	1,103	1,758
Other operating (income) expense, net	(1,263)	(269)	(1,228)	3,301
Total costs and expenses	243,532	207,566	422,709	357,252
Operating income	116,408	114,481	198,644	162,777
Interest expense	10,322	9,270	20,350	18,764
Income before income taxes	106,086	105,211	178,294	144,013
Income taxes	40,961	41,917	68,904	57,368
Net income	$ 65,125	$ 63,294	$ 109,390	$ 86,645