Capitalization and Short-Term Borrowings (Summary of Changes in Common Stock Equity) (Details) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2020	Mar. 31, 2019
Schedule of Capitalization, Equity [Line Items]
Beginning balance (shares)				86,315,287
Beginning balance				$ 832,264
Balance at Beginning of Period		$ 1,320,592	$ 1,172,334	1,272,601	$ 1,098,900
Balance at Beginning of Period		(56,150)	(28,690)	(52,155)	(67,750)
Net Income (Loss) Available for Common Stock		(106,068)	90,595	(19,477)	193,256
Dividends Declared on Common Stock		(37,654)	(36,678)	(75,304)	(73,342)
Other Comprehensive Income (Loss), Net of Tax		$ 37,233	(25,596)	$ 33,238	13,464
Ending balance (shares)		86,561,532		86,561,532
Ending balance		$ 835,444		$ 835,444
Balance at March 31		1,176,870	1,236,657	1,176,870	1,236,657
Balance at End of Period		(18,917)	(54,286)	(18,917)	(54,286)
Paid-in Capital [Member]
Schedule of Capitalization, Equity [Line Items]
Beginning balance		831,146	817,076	832,264	820,223
Share-Based Payment Expense	[1]	3,876	5,038	6,704	9,955
Common Stock Issued Under Stock and Benefit Plans (Value)		422
Common Stock Repurchased under Stock and Benefit Plans			(277)	(3,524)	(8,341)
Ending balance		835,444	821,837	835,444	821,837
Earnings Reinvested in The Business [Member]
Schedule of Capitalization, Equity [Line Items]
Balance at Beginning of Period		1,320,592	1,172,334	1,272,601	1,098,900
Net Income (Loss) Available for Common Stock		(106,068)	90,595	(19,477)	193,256
Dividends Declared on Common Stock		(37,654)	(36,678)	75,304	73,342
Balance at March 31		1,176,870	1,236,657	1,176,870	1,236,657
Accumulated Other Comprehensive Income (Loss) [Member]
Schedule of Capitalization, Equity [Line Items]
Balance at Beginning of Period		(56,150)	(28,690)	(52,155)	(67,750)
Other Comprehensive Income (Loss), Net of Tax		37,233	(25,596)	33,238	13,464
Balance at End of Period		$ (18,917)	$ (54,286)	$ (18,917)	$ (54,286)
Common Stock [Member]
Schedule of Capitalization, Equity [Line Items]
Beginning balance (shares)		86,552,000	86,271,000	86,315,000	85,957,000
Beginning balance (value)		$ 86,552	$ 86,271	$ 86,315	$ 85,957
Common Stock Issued Under Stock and Benefit Plans (Shares)		10,000	30,000	247,000	344,000
Common Stock Issued Under Stock and Benefit Plans (Value)		$ 10	$ 30	$ 247	$ 344
Ending balance (shares)		86,562,000	86,301,000	86,562,000	86,301,000
Ending balance (value)		$ 86,562	$ 86,301	$ 86,562	$ 86,301
Guidance for Hedging [Member]
Schedule of Capitalization, Equity [Line Items]
Cumulative Effect of Adoption of Authoritative Guidance		(1,000)		(1,000)
Guidance for Hedging [Member] | Earnings Reinvested in The Business [Member]
Schedule of Capitalization, Equity [Line Items]
Cumulative Effect of Adoption of Authoritative Guidance		$ (950)		$ (950)
Guidance for Recognition and Measurement of Financial Assets and Liabilities [Member]
Schedule of Capitalization, Equity [Line Items]
Cumulative Effect of Adoption of Authoritative Guidance			7,400		7,400
Guidance for Recognition and Measurement of Financial Assets and Liabilities [Member] | Earnings Reinvested in The Business [Member]
Schedule of Capitalization, Equity [Line Items]
Cumulative Effect of Adoption of Authoritative Guidance			7,437		7,437
Guidance for Reclassification of Stranded Tax Effects [Member]
Schedule of Capitalization, Equity [Line Items]
Cumulative Effect of Adoption of Authoritative Guidance			10,400		10,400
Guidance for Reclassification of Stranded Tax Effects [Member] | Earnings Reinvested in The Business [Member]
Schedule of Capitalization, Equity [Line Items]
Cumulative Effect of Adoption of Authoritative Guidance			$ 10,406		$ 10,406

[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.