Consolidated Statements of Income - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Income Statement [Abstract]
Revenues	$ 1,092,381	$ 887,588	$ 768,484
Costs and expenses:
Cost of sales — gas and purchased power (excluding depreciation shown below)	522,911	367,279	289,786
Operating and administrative expenses	235,334	215,645	196,631
Operating and administrative expenses — related parties	14,234	12,354	11,863
Depreciation	84,644	72,332	67,303
Other operating (income) expense, net	(2,264)	(8,329)	2,000
Costs and expenses	854,859	659,281	567,583
Operating income	237,522	228,307	200,901
Interest expense	42,890	40,212	37,630
Income before income taxes	194,632	188,095	163,271
Income taxes	45,711	72,054	65,898
Net income	$ 148,921	$ 116,041	$ 97,373