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Segment Information (Details) $ in Thousands	3 Months Ended								12 Months Ended
Segment Information (Details) $ in Thousands	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Sep. 30, 2018 USD ($) county Segment customer	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)
Measurement Disclosures
Number of reportable segments (in segments) \| Segment									2
Number of counties \| county									1
Revenues	$ 126,081	$ 159,934	$ 483,261	$ 323,105	$ 119,543	$ 146,692	$ 359,940	$ 261,413	$ 1,092,381	$ 887,588	$ 768,484
Cost of sales									522,911	367,279	289,786
Depreciation									84,644	72,332	67,303
Operating income	2,186	$ 3,914	$ 135,127	$ 96,295	1,992	$ 27,671	$ 116,408	$ 82,236	237,522	228,307	200,901
Interest expense									42,890	40,212	37,630
Income before income taxes									194,632	188,095	163,271
Total assets	2,994,015				2,743,091				3,266,576	2,994,015	2,743,091
Goodwill	182,145				182,145				182,145	182,145	182,145
Capital expenditures (including the effects of accruals)									$ 338,548	317,722	262,503
Electric Utility
Measurement Disclosures
Number of counties \| county									2
Operating Segments \| Gas Utility
Measurement Disclosures
Revenues									$ 994,814	799,054	677,387
Cost of sales									467,528	318,210	239,163
Depreciation									79,124	67,357	62,451
Operating income									230,430	219,561	189,412
Interest expense									41,414	38,218	35,786
Income before income taxes									189,016	181,343	153,626
Total assets	2,833,423				2,570,297				3,075,992	2,833,423	2,570,297
Goodwill	182,145				182,145				182,145	182,145	182,145
Capital expenditures (including the effects of accruals)									320,018	306,243	251,261
Operating Segments \| Electric Utility
Measurement Disclosures
Revenues									97,567	88,534	91,097
Cost of sales									55,383	49,069	50,623
Depreciation									5,520	4,975	4,852
Operating income									7,092	8,746	11,489
Interest expense									1,476	1,994	1,844
Income before income taxes									5,616	6,752	9,645
Total assets	160,592				172,794				190,584	160,592	172,794
Goodwill	$ 0				$ 0				0	0	0
Capital expenditures (including the effects of accruals)									$ 18,530	$ 11,479	$ 11,242
Customer Concentration Risk \| Revenue, Consolidated
Measurement Disclosures
Number of customers \| customer									0