Segment Information - Schedule of Segment Information (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2018	Mar. 31, 2017	Sep. 30, 2017
Segment Reporting Information
Revenues	$ 483,261	$ 359,940	$ 806,366	$ 621,353
Cost of sales — gas, fuel and purchased power	257,302	164,541	409,076	274,012
Depreciation and amortization	21,158	17,699	41,512	35,090
Operating income	135,127	116,408	231,422	198,644
Interest expense	11,091	10,322	22,030	20,350
Income before income taxes	124,036	106,086	209,392	178,294
Capital expenditures (including the effects of accruals)	55,089	56,517	126,788	120,613
Total assets	3,204,045	2,909,725	3,204,045	2,909,725	$ 2,994,015
Goodwill	182,145	182,145	182,145	182,145	$ 182,145
Gas Utility
Segment Reporting Information
Revenues	455,973	335,864	755,938	572,964
Cost of sales — gas, fuel and purchased power	241,031	150,863	379,889	246,430
Depreciation and amortization	19,776	16,378	38,776	32,533
Operating income	134,175	115,105	227,856	194,072
Interest expense	10,866	9,833	21,392	19,416
Income before income taxes	123,309	105,272	206,464	174,656
Capital expenditures (including the effects of accruals)	51,363	54,137	120,205	115,879
Total assets	3,033,352	2,746,144	3,033,352	2,746,144
Goodwill	182,145	182,145	182,145	182,145
Electric Utility
Segment Reporting Information
Revenues	27,288	24,076	50,428	48,389
Cost of sales — gas, fuel and purchased power	16,271	13,678	29,187	27,582
Depreciation and amortization	1,382	1,321	2,736	2,557
Operating income	952	1,303	3,566	4,572
Interest expense	225	489	638	934
Income before income taxes	727	814	2,928	3,638
Capital expenditures (including the effects of accruals)	3,726	2,380	6,583	4,734
Total assets	170,693	163,581	170,693	163,581
Goodwill	$ 0	$ 0	$ 0	$ 0