Condensed Consolidated Statements of Income (unaudited) - USD ($) $ in Thousands	3 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Income Statement [Abstract]
Revenues	$ 323,105	$ 261,413
Costs and expenses:
Cost of sales — gas, fuel and purchased power (excluding depreciation shown below)	151,774	109,471
Operating and administrative expenses	51,984	49,716
Operating and administrative expenses — related parties	2,689	2,564
Depreciation and amortization	20,354	17,391
Other operating expense, net	9	35
Total costs and expenses	226,810	179,177
Operating income	96,295	82,236
Interest expense	10,939	10,028
Income before income taxes	85,356	72,208
Income taxes	17,053	27,943
Net income	$ 68,303	$ 44,265