Segment Information (Details) $ in Thousands	3 Months Ended								12 Months Ended
	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Sep. 30, 2015 USD ($)	Jun. 30, 2015 USD ($)	Mar. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Sep. 30, 2016 USD ($) county Segment customer	Sep. 30, 2015 USD ($)	Sep. 30, 2014 USD ($)
Measurement Disclosures
Number of reportable segments (in segments) | Segment									2
Number of counties | county									1
Revenues	$ 108,172	$ 140,283	$ 322,047	$ 197,982	$ 110,212	$ 143,490	$ 500,573	$ 287,306	$ 768,484	$ 1,041,581	$ 1,086,889
Cost of sales									289,786	510,784	562,942
Depreciation and amortization									67,303	63,590	59,219
Operating income	8,309	$ 29,815	$ 114,481	$ 48,296	3,144	$ 20,184	$ 142,699	$ 75,640	200,901	241,667	246,400
Interest expense									37,630	41,128	38,471
Income before income taxes									163,271	200,539	207,929
Total assets	2,743,091				2,505,984				2,743,091	2,505,984	2,352,143
Goodwill	182,145				182,145				182,145	182,145	182,145
Capital expenditures									$ 262,503	197,684	164,180
Electric Utility
Measurement Disclosures
Number of counties | county									2
Operating Segments | Gas Utility
Measurement Disclosures
Revenues									$ 677,387	933,080	977,333
Cost of sales									239,163	448,617	496,762
Depreciation and amortization									62,451	58,974	54,816
Operating income									189,412	226,485	236,219
Interest expense									35,786	39,112	36,602
Income before income taxes									153,626	187,373	199,617
Total assets	2,570,297				2,360,156				2,570,297	2,360,156	2,211,618
Goodwill	182,145				182,145				182,145	182,145	182,145
Capital expenditures									251,261	189,671	156,425
Operating Segments | Electric Utility
Measurement Disclosures
Revenues									91,097	107,577	108,072
Cost of sales									50,623	62,167	66,180
Depreciation and amortization									4,852	4,616	4,403
Operating income									11,489	14,153	9,668
Interest expense									1,844	2,016	1,869
Income before income taxes									9,645	12,137	7,799
Total assets	172,794				145,828				172,794	145,828	140,525
Goodwill	$ 0				0				0	0	0
Capital expenditures									$ 11,242	8,013	7,755
Other
Measurement Disclosures
Revenues										924	1,484
Cost of sales										0	0
Depreciation and amortization										0	0
Operating income										1,029	513
Interest expense										0	0
Income before income taxes										1,029	513
Total assets					0					0	0
Goodwill					$ 0					0	0
Capital expenditures										$ 0	$ 0
Customer Concentration Risk | Revenue, Consolidated
Measurement Disclosures
Number of customers | customer									0