Segment Information - Schedule of Segment Information (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Sep. 30, 2018
Segment Reporting Information
Revenues	$ 163,893	$ 159,934	$ 916,210	$ 966,300
Cost of sales	61,021	72,537	438,516	481,613
Depreciation	23,141	21,414	67,956	62,926
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
Capital expenditures (including the effects of accruals)	84,451	79,704	232,569	206,492
Total assets	3,375,622	3,194,025	3,375,622	3,194,025	$ 3,266,576
Goodwill	182,145	182,145	182,145	182,145	$ 182,145
Gas Utility
Segment Reporting Information
Revenues	144,441	138,597	842,998	894,535
Cost of sales	51,626	60,837	398,910	440,726
Depreciation	21,612	20,011	63,554	58,787
Operating income	18,592	3,126	208,756	232,049
Pension and other postretirement plans non-service income	387	(498)	1,094	(1,565)
Interest expense	(11,522)	(9,829)	(34,362)	(31,221)
Income (loss) before income taxes	7,457	(7,201)	175,488	199,263
Capital expenditures (including the effects of accruals)	79,272	76,546	221,112	196,751
Total assets	3,191,316	3,008,441	3,191,316	3,008,441
Goodwill	182,145	182,145	182,145	182,145
Electric Utility
Segment Reporting Information
Revenues	19,452	21,337	73,212	71,765
Cost of sales	9,395	11,700	39,606	40,887
Depreciation	1,529	1,403	4,402	4,139
Operating income	1,733	1,357	8,509	5,075
Pension and other postretirement plans non-service income	53	(71)	153	(223)
Interest expense	(803)	(174)	(1,932)	(812)
Income (loss) before income taxes	983	1,112	6,730	4,040
Capital expenditures (including the effects of accruals)	5,179	3,158	11,457	9,741
Total assets	184,306	185,584	184,306	185,584
Goodwill	$ 0	$ 0	$ 0	$ 0