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, 2021	Mar. 31, 2020	Mar. 31, 2021	Mar. 31, 2020
Schedule of Capitalization, Equity [Line Items]
Beginning balance (shares)				90,954,696
Beginning balance				$ 1,004,158
Balance at Beginning of Period		$ 1,028,844	$ 1,320,592	991,630	$ 1,272,601
Balance at Beginning of Period		(79,741)	(56,150)	(114,757)	(52,155)
Net Income (Loss) Available for Common Stock		112,436	(106,068)	190,210	(19,477)
Dividends Declared on Common Stock		(40,562)	(37,654)	(81,122)	(75,304)
Other Comprehensive Income (Loss), Net of Tax		$ (22,247)	37,233	$ 12,769	33,238
Ending balance (shares)		91,163,797		91,163,797
Ending balance		$ 1,009,075		$ 1,009,075
Balance at March 31		1,100,718	1,176,870	1,100,718	1,176,870
Balance at End of Period		$ (101,988)	$ (18,917)	$ (101,988)	$ (18,917)
Dividends per share		$ 0.445	$ 0.435	$ 0.89	$ 0.87
Paid-in Capital [Member]
Schedule of Capitalization, Equity [Line Items]
Beginning balance		$ 1,004,369	$ 831,146	$ 1,004,158	$ 832,264
Share-Based Payment Expense	[1]	4,283	3,876	7,779	6,704
Common Stock Issued Under Stock and Benefit Plans		423	422
Stock Repurchased under Stock and Benefit Plans				(2,862)	(3,524)
Ending balance		1,009,075	835,444	1,009,075	835,444
Earnings Reinvested in The Business [Member]
Schedule of Capitalization, Equity [Line Items]
Balance at Beginning of Period		1,028,844	1,320,592	991,630	1,272,601
Net Income (Loss) Available for Common Stock		112,436	(106,068)	190,210	(19,477)
Dividends Declared on Common Stock		(40,562)	(37,654)	(81,122)	(75,304)
Balance at March 31		1,100,718	1,176,870	1,100,718	1,176,870
Accumulated Other Comprehensive Income (Loss) [Member]
Schedule of Capitalization, Equity [Line Items]
Balance at Beginning of Period		(79,741)	(56,150)	(114,757)	(52,155)
Other Comprehensive Income (Loss), Net of Tax		(22,247)	37,233	12,769	33,238
Balance at End of Period		$ (101,988)	$ (18,917)	$ (101,988)	$ (18,917)
Common Stock [Member]
Schedule of Capitalization, Equity [Line Items]
Beginning balance (shares)		91,153,000	86,552,000	90,955,000	86,315,000
Beginning balance (value)		$ 91,153	$ 86,552	$ 90,955	$ 86,315
Common Stock Issued Under Stock and Benefit Plans (Shares)		11,000	10,000	209,000	247,000
Common Stock Issued Under Stock and Benefit Plans		$ 11	$ 10	$ 209	$ 247
Ending balance (shares)		91,164,000	86,562,000	91,164,000	86,562,000
Ending balance (value)		$ 91,164	$ 86,562	$ 91,164	$ 86,562
Guidance for Hedging [Member] | Cumulative Effect of Adoption of Authoritative Guidance [Member]
Schedule of Capitalization, Equity [Line Items]
Balance at Beginning of Period		$ 0	$ 0	$ 0	(950)
Guidance for Hedging [Member] | Earnings Reinvested in The Business [Member] | Cumulative Effect of Adoption of Authoritative Guidance [Member]
Schedule of Capitalization, Equity [Line Items]
Balance at Beginning of Period					$ (950)

[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.