Condensed Consolidated Statements of Income (unaudited) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Income Statement [Abstract]
Revenues	$ 163,893	$ 159,934	$ 916,210	$ 966,300
Costs and expenses:
Cost of sales — gas and purchased power (excluding depreciation shown below)	61,021	72,537	438,516	481,613
Operating and administrative expenses	56,525	57,656	179,397	175,475
Operating and administrative expenses — related parties	2,861	4,325	11,571	10,780
Depreciation	23,141	21,414	67,956	62,926
Other operating expense (income), net	20	(481)	1,505	(1,618)
Total costs and expenses	143,568	155,451	698,945	729,176
Operating income	20,325	4,483	217,265	237,124
Pension and other postretirement plans non-service income	440	(569)	1,247	(1,788)
Interest expense	(12,325)	(10,003)	(36,294)	(32,033)
Income (loss) before income taxes	8,440	(6,089)	182,218	203,303
Income tax (expense) benefit	(1,767)	3,066	(42,797)	(48,839)
Net income (loss)	$ 6,673	$ (3,023)	$ 139,421	$ 154,464