Segment Information - Schedule of Segment Information (Details) - USD ($) $ in Thousands	3 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Sep. 30, 2017
Segment Reporting Information
Revenues	$ 323,105	$ 261,413
Cost of sales — gas, fuel and purchased power	151,774	109,471
Depreciation and amortization	20,354	17,391
Operating income	96,295	82,236
Interest expense	10,939	10,028
Income before income taxes	85,356	72,208
Capital expenditures (including the effects of accruals)	71,699	64,096
Total assets	3,174,693	2,898,523	$ 2,994,015
Goodwill	182,145	182,145	$ 182,145
Gas Utility
Segment Reporting Information
Revenues	299,965	237,100
Cost of sales — gas, fuel and purchased power	138,858	95,567
Depreciation and amortization	19,000	16,155
Operating income	93,681	78,967
Interest expense	10,526	9,583
Income before income taxes	83,155	69,384
Capital expenditures (including the effects of accruals)	68,842	61,742
Total assets	3,038,250	2,736,908
Goodwill	182,145	182,145
Electric Utility
Segment Reporting Information
Revenues	23,140	24,313
Cost of sales — gas, fuel and purchased power	12,916	13,904
Depreciation and amortization	1,354	1,236
Operating income	2,614	3,269
Interest expense	413	445
Income before income taxes	2,201	2,824
Capital expenditures (including the effects of accruals)	2,857	2,354
Total assets	136,443	161,615
Goodwill	$ 0	$ 0