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Shareholders' Equity Statement of Shareholders' Equity (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended				6 Months Ended
	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2019	Mar. 31, 2018	Sep. 30, 2018	Sep. 30, 2017
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Common stock outstanding (in shares)	116,982,903				116,982,903		111,273,683
Shareholders' equity, beginning balance	$ 5,348,195	$ 4,769,951	$ 4,563,620	$ 3,898,666	$ 4,769,951	$ 3,898,666
Net Income	214,888	157,646	178,992	314,132	372,534	493,124
Other Comprehensive Loss, Net of Tax	(2,695)	(22,258)	21,305	(1,062)
Cash dividends	(61,606)	(58,722)	(54,054)	(51,837)
Stock Issued or Granted During Period, Share-based Compensation [Abstract]
Public and other stock offerings	5,454	498,975	6,235	400,759
Stock-based compensation plans	3,865	2,603	5,248	2,962
Shareholders' equity, ending balance	$ 5,508,101	$ 5,348,195	$ 4,721,346	$ 4,563,620	$ 5,508,101	$ 4,721,346
Cash dividends per share	$ 0.525	$ 0.525	$ 0.485	$ 0.485	$ 1.050	$ 0.970
Common Stock [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Common stock outstanding (in shares)	116,982,903	116,892,959	111,060,328	110,962,112	116,982,903	111,060,328	111,273,683	106,104,634
Shareholders' equity, beginning balance	$ 584	$ 556	$ 555	$ 531	$ 556	$ 531
Stock Issued or Granted During Period, Share-based Compensation [Abstract]
Public and other stock offerings (in shares)	61,006	5,434,812	76,776	4,621,518
Public and other stock offerings	$ 1	$ 27	$ 0	$ 22
Stock-based compensation plans (in shares)	28,938	184,464	21,440	235,960
Stock-based compensation plans	$ 0	$ 1	$ 0	$ 2
Shareholders' equity, ending balance	585	584	555	555	585	555
Additional Paid-in Capital [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shareholders' equity, beginning balance	3,476,476	2,974,926	2,940,062	2,536,365	2,974,926	2,536,365
Stock Issued or Granted During Period, Share-based Compensation [Abstract]
Public and other stock offerings	5,453	498,948	6,235	400,737
Stock-based compensation plans	3,865	2,602	5,248	2,960
Shareholders' equity, ending balance	3,485,794	3,476,476	2,951,545	2,940,062	3,485,794	2,951,545
Accumulated Other Comprehensive Income
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shareholders' equity, beginning balance	(114,115)	(83,647)	(106,316)	(105,254)	(83,647)	(105,254)
Other Comprehensive Loss, Net of Tax	(2,695)	(22,258)	21,305	(1,062)	(24,953)	20,243
Cumulative Effect of New Accounting Principle in Period of Adoption		(8,210)
Stock Issued or Granted During Period, Share-based Compensation [Abstract]
Shareholders' equity, ending balance	(116,810)	(114,115)	(85,011)	(106,316)	(116,810)	(85,011)
Retained Earnings [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shareholders' equity, beginning balance	1,985,250	1,878,116	1,729,319	1,467,024	1,878,116	1,467,024
Net Income	214,888	157,646	178,992	314,132
Cash dividends	(61,606)	(58,722)	(54,054)	(51,837)
Cumulative Effect of New Accounting Principle in Period of Adoption		8,210
Stock Issued or Granted During Period, Share-based Compensation [Abstract]
Shareholders' equity, ending balance	$ 2,138,532	$ 1,985,250	$ 1,854,257	$ 1,729,319	$ 2,138,532	$ 1,854,257