Capitalization and Short-Term Borrowings (Summary of Changes in Common Stock Equity) (Details) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2019	Mar. 31, 2018
Schedule of Capitalization, Equity [Line Items]
Beginning balance (shares)				85,956,814
Beginning balance				$ 820,223
Balance at Beginning of Period		$ 1,172,334	$ 1,014,733	1,098,900	$ 851,669
Balance at Beginning of Period		(28,690)	(40,919)	(67,750)	(30,123)
Net Income Available for Common Stock		90,595	91,847	193,256	290,501
Dividends Declared on Common Stock		(36,678)	(35,641)	(73,342)	(71,231)
Other Comprehensive Income (Loss), Net of Tax		$ (25,596)	(6,841)	$ 13,464	(17,637)
Ending balance (shares)		86,300,675		86,300,675
Ending balance		$ 821,837		$ 821,837
Balance at March 31		1,236,657	1,070,939	1,236,657	1,070,939
Balance at End of Period		(54,286)	(47,760)	(54,286)	(47,760)
Paid-in Capital [Member]
Schedule of Capitalization, Equity [Line Items]
Beginning balance		817,076	800,348	820,223	796,646
Share-Based Payment Expense	[1]	5,038	3,563	9,955	7,074
Common Stock Issued Under Stock and Benefit Plans (Value)			6,215		6,406
Common Stock Repurchased Under Stock and Benefit Plans, Value		(277)		(8,341)
Ending balance		821,837	810,126	821,837	810,126
Earnings Reinvested in The Business [Member]
Schedule of Capitalization, Equity [Line Items]
Balance at Beginning of Period		1,172,334	1,014,733	1,098,900	851,669
Net Income Available for Common Stock		90,595	91,847	193,256	290,501
Dividends Declared on Common Stock		(36,678)	(35,641)	(73,342)	(71,231)
Balance at March 31		1,236,657	1,070,939	1,236,657	1,070,939
Accumulated Other Comprehensive Income (Loss) [Member]
Schedule of Capitalization, Equity [Line Items]
Balance at Beginning of Period		(28,690)	(40,919)	(67,750)	(30,123)
Other Comprehensive Income (Loss), Net of Tax		(25,596)	(6,841)	13,464	(17,637)
Balance at End of Period		$ (54,286)	$ (47,760)	$ (54,286)	$ (47,760)
Common Stock [Member]
Schedule of Capitalization, Equity [Line Items]
Beginning balance (shares)		86,271,000	85,761,000	85,957,000	85,543,000
Beginning balance (value)		$ 86,271	$ 85,761	$ 85,957	$ 85,543
Common Stock Issued Under Stock and Benefit Plans (Shares)		30,000	121,000	344,000	339,000
Common Stock Issued Under Stock and Benefit Plans (Value)		$ 30	$ 121	$ 344	$ 339
Ending balance (shares)		86,301,000	85,882,000	86,301,000	85,882,000
Ending balance (value)		$ 86,301	$ 85,882	$ 86,301	$ 85,882
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
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

[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.