Segment Information (Details) $ in Thousands	3 Months Ended								12 Months Ended
	Sep. 30, 2015 USD ($)	Jun. 30, 2015 USD ($)	Mar. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Sep. 30, 2014 USD ($)	Jun. 30, 2014 USD ($)	Mar. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Sep. 30, 2015 USD ($) county Segment customer	Sep. 30, 2014 USD ($)	Sep. 30, 2013 USD ($)
Segment Reporting Information
Revenues	$ 110,212	$ 143,490	$ 500,573	$ 287,306	$ 121,340	$ 152,694	$ 513,956	$ 298,899	$ 1,041,581	$ 1,086,889	$ 940,712
Cost of sales									510,784	562,942	465,996
Depreciation and amortization									63,590	59,219	55,716
Operating income	3,144	$ 20,184	$ 142,699	$ 75,640	2,883	$ 19,720	$ 137,954	$ 85,843	241,667	246,400	210,319
Interest expense									41,128	38,471	39,309
Income before income taxes									200,539	207,929	171,010
Total assets	2,508,178				2,354,643				2,508,178	2,354,643	2,210,322
Goodwill	182,145				182,145				182,145	182,145	182,145
Capital expenditures									$ 197,684	164,180	151,090
Measurement Disclosures
Number of reportable segments (in segments) | Segment									2
Number of counties | county									1
Customer Concentration Risk | Revenue, Consolidated
Measurement Disclosures
Number of customers | customer									0
Electric Utility
Measurement Disclosures
Number of counties | county									2
Operating Segments | Gas Utility
Segment Reporting Information
Revenues									$ 933,080	977,333	839,050
Cost of sales									448,617	496,762	407,222
Depreciation and amortization									58,974	54,816	51,698
Operating income									226,485	236,219	198,352
Interest expense									39,112	36,602	37,280
Income before income taxes									187,373	199,617	161,072
Total assets	2,362,350				2,214,118				2,362,350	2,214,118	2,068,955
Goodwill	182,145				182,145				182,145	182,145	182,145
Capital expenditures									189,671	156,425	144,399
Operating Segments | Electric Utility
Segment Reporting Information
Revenues									107,577	108,072	99,986
Cost of sales									62,167	66,180	58,774
Depreciation and amortization									4,616	4,403	4,018
Operating income									14,153	9,668	11,385
Interest expense									2,016	1,869	2,029
Income before income taxes									12,137	7,799	9,356
Total assets	145,828				140,525				145,828	140,525	141,367
Goodwill	0				0				0	0	0
Capital expenditures									8,013	7,755	6,691
Segment Reconciling Items
Segment Reporting Information
Revenues									924	1,484	1,676
Cost of sales									0	0	0
Depreciation and amortization									0	0	0
Operating income									1,029	513	582
Interest expense									0	0	0
Income before income taxes									1,029	513	582
Total assets	0				0				0	0	0
Goodwill	$ 0				$ 0				0	0	0
Capital expenditures									$ 0	$ 0	$ 0