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, 2024	Mar. 31, 2023	Mar. 31, 2024	Mar. 31, 2023
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance (in shares)				91,819,405
Beginning balance				$ 2,963,376
Net Income Available for Common Stock		$ 166,272	$ 140,880	299,292	$ 310,570
Dividends Declared on Common Stock		(45,563)	(43,602)	(91,160)	(87,201)
Other Comprehensive Income, Net of Tax		$ 7,959	$ 238,882	$ 130,400	$ 570,869
Ending balance (in shares)		92,031,724		92,031,724
Ending balance		$ 3,303,473		$ 3,303,473
Dividends per share (in dollars per share)		$ 0.495	$ 0.475	$ 0.99	$ 0.95
Stock and Benefit Plans
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Share Repurchases (in shares)				(77,426)
Share Repurchase Plan
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Share Repurchases (in shares)		(96,133)
Share Repurchases		$ (5,000)
Common Stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance (in shares)		92,116,000	91,787,000	91,819,000	91,478,000
Beginning balance		$ 92,116	$ 91,787	$ 91,819	$ 91,478
Common stock issued under stock and benefit plans (in shares)		12,000	8,000	309,000	317,000
Common Stock Issued Under Stock and Benefit Plans		$ 12	$ 8	$ 309	$ 317
Ending balance (in shares)		92,032,000	91,795,000	92,032,000	91,795,000
Ending balance		$ 92,032	$ 91,795	$ 92,032	$ 91,795
Common Stock | Share Repurchase Plan
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Share Repurchases (in shares)		(96,000)		(96,000)
Share Repurchases		$ (96)		$ (96)
Paid In Capital
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance		1,041,226	1,025,639	1,040,761	1,027,066
Share-Based Payment Expense	[1]	5,222	5,200	9,356	10,318
Common Stock Issued Under Stock and Benefit Plans		569	502
Ending balance		1,045,929	1,031,341	1,045,929	1,031,341
Paid In Capital | Stock and Benefit Plans
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Share Repurchases				(3,100)	(6,043)
Paid In Capital | Share Repurchase Plan
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Share Repurchases		(1,088)		(1,088)
Earnings Reinvested in the Business
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance		1,973,279	1,713,176	1,885,856	1,587,085
Net Income Available for Common Stock		166,272	140,880	299,292	310,570
Dividends Declared on Common Stock		(45,563)	(43,602)	(91,160)	(87,201)
Share Repurchases		(3,816)	0	(3,816)	0
Ending balance		2,090,172	1,810,454	2,090,172	1,810,454
Earnings Reinvested in the Business | Share Repurchase Plan
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Share Repurchases		(3,816)		(3,816)
Accumulated Other Comprehensive Income (Loss)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance		67,381	(293,746)	(55,060)	(625,733)
Other Comprehensive Income, Net of Tax		7,959	238,882	130,400	570,869
Ending balance		$ 75,340	$ (54,864)	$ 75,340	$ (54,864)

[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.