Segment Information - Schedule of Segment Information (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015	Sep. 30, 2015
Segment Reporting Information
Revenues	$ 140,283	$ 143,490	$ 660,312	$ 931,369
Cost of sales	44,415	53,691	257,288	475,079
Depreciation and amortization	16,550	15,913	50,281	46,982
Operating income	29,815	20,184	192,592	238,523
Interest expense	9,158	9,985	27,922	31,245
Income before income taxes	20,657	10,199	164,670	207,278
Capital expenditures (including the effects of accruals)	56,481	43,315	166,058	139,624
Total assets (at period end)	2,699,898	2,423,205	2,699,898	2,423,205	$ 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	119,995	119,412	595,025	847,890
Cost of sales	33,715	41,352	221,646	426,715
Depreciation and amortization	15,339	14,786	46,665	43,555
Operating income	27,116	15,099	183,940	226,248
Interest expense	8,670	9,483	26,583	29,717
Income before income taxes	18,446	5,616	157,357	196,531
Capital expenditures (including the effects of accruals)	53,199	41,324	158,472	134,018
Total assets (at period end)	2,534,039	2,278,975	2,534,039	2,278,975
Goodwill (at period end)	182,145	182,145	182,145	182,145
Reportable Segments | Electric Utility
Segment Reporting Information
Revenues	20,288	23,875	65,287	82,621
Cost of sales	10,700	12,339	35,642	48,364
Depreciation and amortization	1,211	1,127	3,616	3,427
Operating income	2,699	4,071	8,652	11,300
Interest expense	488	502	1,339	1,528
Income before income taxes	2,211	3,569	7,313	9,772
Capital expenditures (including the effects of accruals)	3,282	1,991	7,586	5,606
Total assets (at period end)	165,859	144,230	165,859	144,230
Goodwill (at period end)	$ 0	0	$ 0	0
Other
Segment Reporting Information
Revenues		203		858
Cost of sales		0		0
Depreciation and amortization		0		0
Operating income		1,014		975
Interest expense		0		0
Income before income taxes		1,014		975
Capital expenditures (including the effects of accruals)		0		0
Total assets (at period end)		0		0
Goodwill (at period end)		$ 0		$ 0