Segment Information - Schedule of Segment Information (Details) - USD ($) $ in Thousands	3 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Sep. 30, 2015
Segment Reporting Information
Revenues	$ 197,982	$ 287,306
Cost of sales	75,439	143,052
Depreciation and amortization	16,701	15,425
Operating income	48,296	75,640
Interest expense	9,494	10,649
Income before income taxes	38,802	64,991
Capital expenditures	61,464	55,029
Total assets (at period end)	2,606,299	2,488,621	$ 2,508,178
Goodwill (at period end)	182,145	182,145	$ 182,145
Reportable Segments | Gas Utility
Segment Reporting Information
Revenues	176,942	260,478
Cost of sales	64,229	127,208
Depreciation and amortization	15,504	14,280
Operating income	45,820	71,846
Interest expense	9,066	10,130
Income before income taxes	36,754	61,716
Capital expenditures	59,270	53,492
Total assets (at period end)	2,462,508	2,346,169
Goodwill (at period end)	182,145	182,145
Reportable Segments | Electric Utility
Segment Reporting Information
Revenues	21,040	26,423
Cost of sales	11,210	15,844
Depreciation and amortization	1,197	1,145
Operating income	2,476	3,719
Interest expense	428	519
Income before income taxes	2,048	3,200
Capital expenditures	2,194	1,537
Total assets (at period end)	143,791	142,452
Goodwill (at period end)	$ 0	0
Other
Segment Reporting Information
Revenues		405
Cost of sales		0
Depreciation and amortization		0
Operating income		75
Interest expense		0
Income before income taxes		75
Capital expenditures		0
Total assets (at period end)		0
Goodwill (at period end)		$ 0