Capitalization and Short-Term Borrowings - Summary of Changes in Common Stock Equity (Details) - USD ($) $ / shares in Units, $ in Thousands		12 Months Ended
		Sep. 30, 2023		Sep. 30, 2022	Sep. 30, 2021
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance (in shares)		91,478,064
Beginning balance		$ 2,079,896
Net Income Available for Common Stock		476,866		$ 566,021	$ 363,647
Dividends Declared on Common Stock		(178,095)		(170,111)	(164,102)
Other Comprehensive Loss, Net of Tax		$ 570,673		$ (112,136)	$ (398,840)
Ending balance (in shares)		91,819,405		91,478,064
Ending balance		$ 2,963,376		$ 2,079,896
Dividend per share (in dollars per share)		$ 1.94		$ 1.86	$ 1.80
Accumulated earnings free from limitations		$ 1,700,000
Common Stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance (in shares)		91,478,000		91,182,000	90,955,000
Beginning balance		$ 91,478		$ 91,182	$ 90,955
Common Stock Issued (Repurchased) Under Stock and Benefit Plans (in shares)		341,000		296,000	227,000
Common Stock Issued (Repurchased) Under Stock and Benefit Plans		$ 341		$ 296	$ 227
Ending balance (in shares)		91,819,000		91,478,000	91,182,000
Ending balance		$ 91,819		$ 91,478	$ 91,182
Paid In Capital
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance		1,027,066		1,017,446	1,004,158
Share-Based Payment Expense	[1]	18,746		17,699	15,297
Common Stock Issued (Repurchased) Under Stock and Benefit Plans		(5,051)		(8,079)	(2,009)
Ending balance		1,040,761		1,027,066	1,017,446
Earnings Reinvested in the Business
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance		1,587,085		1,191,175	991,630
Net Income Available for Common Stock		476,866		566,021	363,647
Dividends Declared on Common Stock		(178,095)		(170,111)	(164,102)
Ending balance		1,885,856	[2]	1,587,085	1,191,175
Accumulated Other Comprehensive Income (Loss)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance		(625,733)		(513,597)	(114,757)
Other Comprehensive Loss, Net of Tax		570,673		(112,136)	(398,840)
Ending balance		$ (55,060)		$ (625,733)	$ (513,597)

[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.

[2] The availability of consolidated earnings reinvested in the business for dividends payable in cash is limited under terms of the indentures covering long-term debt. At September 30, 2023, $1.7 billion of accumulated earnings was free of such limitations.