Condensed Consolidated Statements of Income (unaudited) - USD ($) $ in Thousands	3 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Income Statement [Abstract]
Revenues	$ 261,413	$ 197,982
Costs and expenses:
Cost of sales — gas, fuel and purchased power (excluding depreciation shown below)	109,471	75,439
Operating and administrative expenses	46,037	48,027
Operating and administrative expenses — related parties	2,564	2,180
Taxes other than income taxes	3,679	3,769
Depreciation	16,862	15,827
Amortization	529	874
Other operating expense, net	35	3,570
Total costs and expenses	179,177	149,686
Operating income	82,236	48,296
Interest expense	10,028	9,494
Income before income taxes	72,208	38,802
Income taxes	27,943	15,451
Net income	$ 44,265	$ 23,351