Selected Quarterly Financial Data (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended									12 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Dec. 31, 2014	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Operating Revenues by Segment [Line Items]
Operating revenues	$ 443,683	$ 485,738	$ 1,094,645	$ 877,782	$ 444,700	$ 562,245	$ 1,219,409	$ 889,192
Operating income	89,715	122,202	297,677	236,464	90,629	124,320	270,902	242,083		$ 746,058	$ 727,934	$ 735,628
Net income	$ 58,406	$ 80,466	$ 214,888	$ 157,646	$ 38,747	$ 71,193	$ 178,992	$ 314,132		$ 511,406	$ 603,064	$ 396,421
Basic and diluted earnings per share
Earnings Per Share, Basic	$ 0.49	$ 0.68	$ 1.83	$ 1.38	$ 0.35	$ 0.64	$ 1.60	$ 2.89	$ 3.73	$ 4.36	$ 5.43	$ 3.73
Earnings Per Share, Diluted	$ 0.49	$ 0.68	$ 1.82	$ 1.38	$ 0.35	$ 0.64	$ 1.60	$ 2.89		$ 4.35	$ 5.43	$ 3.73
Distribution
Operating Revenues by Segment [Line Items]
Operating revenues	$ 403,793	$ 444,944	$ 1,057,889	$ 838,835	$ 407,476	$ 535,488	$ 1,199,291	$ 860,792
Pipeline and Storage
Operating Revenues by Segment [Line Items]
Operating revenues	147,706	149,198	135,650	134,470	132,662	127,633	120,955	126,463
Eliminations
Operating Revenues by Segment [Line Items]
Operating revenues	(107,816)	(108,404)	(98,894)	(95,523)	(95,438)	(100,876)	(100,837)	(98,063)
Natural Gas, US Regulated
Operating Revenues by Segment [Line Items]
Total purchased gas cost	$ 13,670	$ 31,326	$ 471,676	$ 342,165	$ 43,085	$ 130,886	$ 626,960	$ 366,917		$ 858,837	$ 1,167,848	$ 925,536