Segment Information - Schedule of Segment Information (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2019	Mar. 31, 2018	Sep. 30, 2018
Segment Reporting Information
Revenues	$ 429,592	$ 483,261	$ 752,317	$ 806,366
Cost of sales	217,976	257,302	377,495	409,076
Depreciation	22,341	21,158	44,815	41,512
Operating income	119,866	135,771	196,940	232,641
Pension and other postretirement plans non-service income	395	(644)	807	(1,219)
Interest expense	(12,231)	(11,091)	(23,969)	(22,030)
Income before income taxes	108,030	124,036	173,778	209,392
Capital expenditures (including the effects of accruals)	70,799	55,089	148,118	126,788
Total assets	3,438,437	3,204,045	3,438,437	3,204,045	$ 3,266,576
Goodwill	182,145	182,145	182,145	182,145	$ 182,145
Gas Utility
Segment Reporting Information
Revenues	401,415	455,973	698,557	755,938
Cost of sales	202,164	241,031	347,284	379,889
Depreciation	20,902	19,776	41,942	38,776
Operating income	116,580	134,738	190,164	228,923
Pension and other postretirement plans non-service income	346	(563)	707	(1,067)
Interest expense	(11,622)	(10,866)	(22,840)	(21,392)
Income before income taxes	105,304	123,309	168,031	206,464
Capital expenditures (including the effects of accruals)	66,766	51,363	141,840	120,205
Total assets	3,252,092	3,033,352	3,252,092	3,033,352
Goodwill	182,145	182,145	182,145	182,145
Electric Utility
Segment Reporting Information
Revenues	28,177	27,288	53,760	50,428
Cost of sales	15,812	16,271	30,211	29,187
Depreciation	1,439	1,382	2,873	2,736
Operating income	3,286	1,033	6,776	3,718
Pension and other postretirement plans non-service income	49	(81)	100	(152)
Interest expense	(609)	(225)	(1,129)	(638)
Income before income taxes	2,726	727	5,747	2,928
Capital expenditures (including the effects of accruals)	4,033	3,726	6,278	6,583
Total assets	186,345	170,693	186,345	170,693
Goodwill	$ 0	$ 0	$ 0	$ 0