Selected Quarterly Financial Data (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Operating Revenues by Segment [Line Items]
Operating revenues	$ 444,700	$ 562,245	$ 1,219,409	$ 889,192	$ 464,880	$ 526,501	$ 988,186	$ 780,168
Operating income	89,592	122,993	268,988	241,561	91,792	140,664	285,172	209,918	$ 723,134	$ 727,546	$ 657,230
Income from continuing operations					35,853	70,808	162,012	114,038	603,064	382,711	345,542
Income from discontinued operations, net of tax, before gain on disposal					0	0	0	10,994
Gain on sale of discontinued operations, net of tax ($0, $10,215 and $0)					0	0	2,716	0	0	2,716	0
Net income	$ 38,747	$ 71,193	$ 178,992	$ 314,132	$ 35,853	$ 70,808	$ 164,728	$ 125,032	$ 603,064	$ 396,421	$ 350,104
Basic and diluted earnings per share
Net income per share - basic and diluted (in dollars per share)	$ 0.35	$ 0.64	$ 1.60	$ 2.89	$ 0.34	$ 0.67	$ 1.55	$ 1.19	$ 5.43	$ 3.73	$ 3.38
Income per share from continuing operations (in dollars per share)					0.34	0.67	1.52	1.08	5.43	3.60	3.33
Income per share from discontinued operations (in dollars per share)					$ 0.00	$ 0.00	$ 0.03	$ 0.11	$ 0.00	$ 0.13	$ 0.05
Distribution
Operating Revenues by Segment [Line Items]
Operating revenues	$ 407,476	$ 535,488	$ 1,199,291	$ 860,792	$ 437,918	$ 494,060	$ 962,541	$ 754,656
Pipeline and Storage
Operating Revenues by Segment [Line Items]
Operating revenues	132,662	127,633	120,955	126,463	117,823	117,283	111,972	109,952
Eliminations
Operating Revenues by Segment [Line Items]
Operating revenues	(95,438)	(100,876)	(100,837)	(98,063)	(90,861)	(84,842)	(86,327)	(84,440)
Natural Gas, US Regulated
Operating Revenues by Segment [Line Items]
Total purchased gas cost	$ 43,085	$ 130,886	$ 626,960	$ 366,917	$ 72,561	$ 114,176	$ 427,494	$ 311,305	$ 1,167,848	$ 925,536	$ 746,192