Segment Information - Schedule of Segment Information (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2016	Mar. 31, 2015	Mar. 31, 2016	Mar. 31, 2015	Sep. 30, 2015
Segment Reporting Information
Revenues	$ 322,047	$ 500,573	$ 520,029	$ 787,879
Cost of sales	137,434	278,336	212,873	421,388
Depreciation and amortization	17,030	15,644	33,731	31,069
Operating income (loss)	114,481	142,699	162,777	218,339
Interest expense	9,270	10,611	18,764	21,260
Income (loss) before income taxes	105,211	132,088	144,013	197,079
Capital expenditures (including the effects of accruals)	48,113	41,280	109,577	96,309
Total assets (at period end)	2,641,587	2,506,043	2,641,587	2,506,043	$ 2,508,178
Goodwill (at period end)	182,145	182,145	182,145	182,145	$ 182,145
Reportable Segments | Gas Utility
Segment Reporting Information
Revenues	298,088	468,000	475,030	728,478
Cost of sales	123,702	258,155	187,931	385,363
Depreciation and amortization	15,822	14,489	31,326	28,769
Operating income (loss)	111,004	139,303	156,824	211,149
Interest expense	8,847	10,104	17,913	20,234
Income (loss) before income taxes	102,157	129,199	138,911	190,915
Capital expenditures (including the effects of accruals)	46,003	39,202	105,273	92,694
Total assets (at period end)	2,488,191	2,359,060	2,488,191	2,359,060
Goodwill (at period end)	182,145	182,145	182,145	182,145
Reportable Segments | Electric Utility
Segment Reporting Information
Revenues	23,959	32,323	44,999	58,746
Cost of sales	13,732	20,181	24,942	36,025
Depreciation and amortization	1,208	1,155	2,405	2,300
Operating income (loss)	3,477	3,510	5,953	7,229
Interest expense	423	507	851	1,026
Income (loss) before income taxes	3,054	3,003	5,102	6,203
Capital expenditures (including the effects of accruals)	2,110	2,078	4,304	3,615
Total assets (at period end)	153,396	146,983	153,396	146,983
Goodwill (at period end)	$ 0	0	$ 0	0
Other
Segment Reporting Information
Revenues		250		655
Cost of sales		0		0
Depreciation and amortization		0		0
Operating income (loss)		(114)		(39)
Interest expense		0		0
Income (loss) before income taxes		(114)		(39)
Capital expenditures (including the effects of accruals)		0		0
Total assets (at period end)		0		0
Goodwill (at period end)		$ 0		$ 0