CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS (UNAUDITED) - USD ($) $ in Thousands	3 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Cash Flows From Operating Activities
Net income	$ 314,132	$ 125,032
Adjustments to reconcile net income to net cash provided by operating activities:
Charged to depreciation and amortization	88,374	77,143
Deferred income taxes	53,149	67,241
One-Time Income Tax Benefit, TCJA	(161,884)	0
Discontinued cash flow hedging for natural gas marketing commodity contracts	0	(10,579)
Other	6,915	4,842
Net assets / liabilities from risk management activities	2,030	3,969
Net change in operating assets and liabilities	(129,478)	(150,685)
Net cash provided by operating activities	173,238	116,963
Cash Flows From Investing Activities
Capital expenditures	(383,238)	(297,962)
Acquisition	0	(85,714)
Available-for-sale securities activities, net	(135)	(10,263)
Other, net	2,001	1,802
Net cash used in investing activities	(381,372)	(392,137)
Cash Flows From Financing Activities
Net increase in short-term debt	(110,929)	110,936
Net proceeds from equity offering	395,099	49,400
Issuance of common stock through stock purchase and employee retirement plans	5,660	8,998
Proceeds from issuance of long-term debt, net of discount	0	125,000
Interest rate agreements cash collateral	0	25,670
Cash dividends paid	(51,837)	(47,740)
Repurchase of equity awards	(1,518)	0
Net cash provided by (used in) financing activities	236,475	272,264
Net increase (decrease) in cash and cash equivalents	28,341	(2,910)
Cash and cash equivalents at beginning of period	26,409	47,534
Cash and cash equivalents at end of period	$ 54,750	$ 44,624