Condensed Consolidated Statements of Income (unaudited) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2016	Mar. 31, 2015	Mar. 31, 2016	Mar. 31, 2015
Income Statement [Abstract]
Revenues	$ 322,047	$ 500,573	$ 520,029	$ 787,879
Costs and expenses:
Cost of sales — gas, fuel and purchased power (excluding depreciation shown below)	137,434	278,336	212,873	421,388
Operating and administrative expenses	45,125	58,917	93,152	105,465
Operating and administrative expenses — related parties	3,798	4,138	5,978	6,920
Taxes other than income taxes	4,448	4,803	8,217	8,907
Depreciation	16,146	14,757	31,973	29,315
Amortization	884	887	1,758	1,754
Other operating (income) loss, net	(269)	(3,964)	3,301	(4,209)
Total costs and expenses	207,566	357,874	357,252	569,540
Operating income	114,481	142,699	162,777	218,339
Interest expense	9,270	10,611	18,764	21,260
Income before income taxes	105,211	132,088	144,013	197,079
Income taxes	41,917	52,499	57,368	78,651
Net income	$ 63,294	$ 79,589	$ 86,645	$ 118,428