Segment Information - Schedule of Segment Information (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Sep. 30, 2017
Segment Reporting Information
Revenues	$ 159,934	$ 146,692	$ 966,300	$ 768,045
Cost of sales — gas and purchased power	72,537	51,979	481,613	325,991
Depreciation and amortization	21,414	17,912	62,926	53,002
Operating income	3,914	27,671	235,336	226,315
Interest expense	10,003	10,128	32,033	30,478
(Loss) income before income taxes	(6,089)	17,543	203,303	195,837
Capital expenditures (including the effects of accruals)	79,704	79,088	206,492	199,701
Total assets	3,194,025	2,904,532	3,194,025	2,904,532	$ 2,994,015
Goodwill	182,145	182,145	182,145	182,145	$ 182,145
Gas Utility
Segment Reporting Information
Revenues	138,597	127,849	894,535	700,813
Cost of sales — gas and purchased power	60,837	42,180	440,726	288,610
Depreciation and amortization	20,011	16,845	58,787	49,378
Operating income	2,628	25,628	230,484	219,700
Interest expense	9,829	9,601	31,221	29,017
(Loss) income before income taxes	(7,201)	16,027	199,263	190,683
Capital expenditures (including the effects of accruals)	76,546	75,836	196,751	191,715
Total assets	3,008,441	2,743,035	3,008,441	2,743,035
Goodwill	182,145	182,145	182,145	182,145
Electric Utility
Segment Reporting Information
Revenues	21,337	18,843	71,765	67,232
Cost of sales — gas and purchased power	11,700	9,799	40,887	37,381
Depreciation and amortization	1,403	1,067	4,139	3,624
Operating income	1,286	2,043	4,852	6,615
Interest expense	174	527	812	1,461
(Loss) income before income taxes	1,112	1,516	4,040	5,154
Capital expenditures (including the effects of accruals)	3,158	3,252	9,741	7,986
Total assets	185,584	161,497	185,584	161,497
Goodwill	$ 0	$ 0	$ 0	$ 0