Selected Quarterly Financial Data (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Operating Revenues by Segment [Line Items]
Operating revenues	$ 474,914	$ 492,995	$ 977,665	$ 875,563	$ 443,683	$ 485,738	$ 1,094,645	$ 877,782
Total purchased gas cost									$ 658,854	$ 858,837	$ 1,167,848
Operating income	100,845	139,035	331,438	252,781	89,715	122,202	297,677	236,464	824,099	746,058	727,934
Net Income	$ 65,333	$ 117,791	$ 239,646	$ 178,673	$ 58,406	$ 80,466	$ 214,888	$ 157,646	$ 601,443	$ 511,406	$ 603,064
Basic and diluted earnings per share
Basic net income per share	$ 0.53	$ 0.96	$ 1.95	$ 1.47	$ 0.49	$ 0.68	$ 1.83	$ 1.38	$ 4.89	$ 4.36	$ 5.43
Diluted net income per share	$ 0.53	$ 0.96	$ 1.95	$ 1.47	$ 0.49	$ 0.68	$ 1.82	$ 1.38	$ 4.89	$ 4.35	$ 5.43
Distribution
Operating Revenues by Segment [Line Items]
Operating revenues	$ 430,176	$ 435,308	$ 933,005	$ 828,504	$ 403,793	$ 444,944	$ 1,057,889	$ 838,835
Pipeline and Storage
Operating Revenues by Segment [Line Items]
Operating revenues	156,918	158,008	146,237	148,176	147,706	149,198	135,650	134,470
Natural Gas, US Regulated
Operating Revenues by Segment [Line Items]
Total purchased gas cost	18,031	26,072	317,883	296,868	13,670	31,326	471,676	342,165	$ 658,854	$ 858,837	$ 1,167,848
Eliminations
Operating Revenues by Segment [Line Items]
Operating revenues	$ (112,180)	$ (100,321)	$ (101,577)	$ (101,117)	$ (107,816)	$ (108,404)	$ (98,894)	$ (95,523)
Total purchased gas cost									(413,921)	(409,394)	(393,966)
Operating income									0	0	0
Eliminations | Natural Gas, US Regulated
Operating Revenues by Segment [Line Items]
Total purchased gas cost									$ (413,921)	$ (409,394)	$ (393,966)