Capitalization and Short-Term Borrowings (Summary of Changes in Common Stock Equity) (Details) - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended		6 Months Ended
		Mar. 31, 2023	Mar. 31, 2022	Mar. 31, 2023	Mar. 31, 2022
Schedule of Capitalization, Equity [Line Items]
Beginning balance (shares)				91,478,064
Beginning balance				$ 1,027,066
Balance at Beginning of Period		$ 1,713,176	$ 1,281,963	1,587,085	$ 1,191,175
Balance at Beginning of Period		(293,746)	(277,026)	(625,733)	(513,597)
Net Income Available for Common Stock		140,880	167,328	310,570	299,720
Dividends Declared on Common Stock		(43,602)	(41,608)	(87,201)	(83,212)
Other Comprehensive Income (Loss), Net of Tax		$ 238,882	(377,228)	$ 570,869	(140,657)
Ending balance (shares)		91,795,080		91,795,080
Ending balance		$ 1,031,341		$ 1,031,341
Balance at March 31		1,810,454	1,407,683	1,810,454	1,407,683
Balance at End of Period		$ (54,864)	$ (654,254)	$ (54,864)	$ (654,254)
Dividends per share		$ 0.475	$ 0.455	$ 0.95	$ 0.91
Paid-in Capital [Member]
Schedule of Capitalization, Equity [Line Items]
Beginning balance		$ 1,025,639	$ 1,013,821	$ 1,027,066	$ 1,017,446
Share-Based Payment Expense	[1]	5,200	4,692	10,318	9,732
Common Stock Issued Under Stock and Benefit Plans		502	271
Stock Repurchased under Stock and Benefit Plans				(6,043)	(8,394)
Ending balance		1,031,341	1,018,784	1,031,341	1,018,784
Earnings Reinvested in The Business [Member]
Schedule of Capitalization, Equity [Line Items]
Balance at Beginning of Period		1,713,176	1,281,963	1,587,085	1,191,175
Net Income Available for Common Stock		140,880	167,328	310,570	299,720
Dividends Declared on Common Stock		(43,602)	(41,608)	(87,201)	(83,212)
Balance at March 31		1,810,454	1,407,683	1,810,454	1,407,683
Accumulated Other Comprehensive Income (Loss) [Member]
Schedule of Capitalization, Equity [Line Items]
Balance at Beginning of Period		(293,746)	(277,026)	(625,733)	(513,597)
Other Comprehensive Income (Loss), Net of Tax		238,882	(377,228)	570,869	(140,657)
Balance at End of Period		$ (54,864)	$ (654,254)	$ (54,864)	$ (654,254)
Common Stock [Member]
Schedule of Capitalization, Equity [Line Items]
Beginning balance (shares)		91,787,000	91,437,000	91,478,000	91,182,000
Beginning balance (value)		$ 91,787	$ 91,437	$ 91,478	$ 91,182
Common Stock Issued Under Stock and Benefit Plans (Shares)		8,000	12,000	317,000	267,000
Common Stock Issued Under Stock and Benefit Plans		$ 8	$ 12	$ 317	$ 267
Ending balance (shares)		91,795,000	91,449,000	91,795,000	91,449,000
Ending balance (value)		$ 91,795	$ 91,449	$ 91,795	$ 91,449

[1] Paid in Capital includes compensation costs associated with performance shares and/or restricted stock awards. The expense is included within Net Income Available For Common Stock, net of tax benefits.