Shareholders' Equity Statement of Shareholders' Equity (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended						9 Months Ended
	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Jun. 30, 2019	Jun. 30, 2018	Oct. 01, 2018	Sep. 30, 2018	Sep. 30, 2017
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Common stock outstanding (in shares)	118,196,113						118,196,113			111,273,683
Shareholders' equity, beginning balance	$ 5,508,101	$ 5,348,195	$ 4,769,951	$ 4,721,346	$ 4,563,620	$ 3,898,666	$ 4,769,951	$ 3,898,666
Net income	80,466	214,888	157,646	71,193	178,992	314,132	453,000	564,317
Other Comprehensive Loss, Net of Tax	1,147	(2,695)	(22,258)	8,630	21,305	(1,062)
Cash dividends	(61,654)	(61,606)	(58,722)	(54,116)	(54,054)	(51,837)
Stock Issued or Granted During Period, Share-based Compensation [Abstract]
Public and other stock offerings	103,430	5,454	498,975	3,948	6,235	400,759
Stock-based compensation plans	10,506	3,865	2,603	8,551	5,248	2,962
Shareholders' equity, ending balance	$ 5,641,996	$ 5,508,101	$ 5,348,195	$ 4,759,552	$ 4,721,346	$ 4,563,620	$ 5,641,996	$ 4,759,552
Cash dividends per share	$ 0.525	$ 0.525	$ 0.525	$ 0.485	$ 0.485	$ 0.485	$ 1.575	$ 1.455
Common Stock [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Common stock outstanding (in shares)	118,196,113	116,982,903	116,892,959	111,195,448	111,060,328	110,962,112	118,196,113	111,195,448		111,273,683	106,104,634
Shareholders' equity, beginning balance	$ 585	$ 584	$ 556	$ 555	$ 555	$ 531	$ 556	$ 531
Stock Issued or Granted During Period, Share-based Compensation [Abstract]
Public and other stock offerings (in shares)	1,127,244	61,006	5,434,812	45,307	76,776	4,621,518
Public and other stock offerings	$ 5	$ 1	$ 27	$ 1	$ 0	$ 22
Stock-based compensation plans (in shares)	85,966	28,938	184,464	89,813	21,440	235,960
Stock-based compensation plans	$ 1	$ 0	$ 1	$ 0	$ 0	$ 2
Shareholders' equity, ending balance	591	585	584	556	555	555	591	556
Additional Paid-in Capital [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shareholders' equity, beginning balance	3,485,794	3,476,476	2,974,926	2,951,545	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	103,425	5,453	498,948	3,947	6,235	400,737
Stock-based compensation plans	10,505	3,865	2,602	8,551	5,248	2,960
Shareholders' equity, ending balance	3,599,724	3,485,794	3,476,476	2,964,043	2,951,545	2,940,062	3,599,724	2,964,043
Accumulated Other Comprehensive Income
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shareholders' equity, beginning balance	(116,810)	(114,115)	(83,647)	(85,011)	(106,316)	(105,254)	(83,647)	(105,254)
Other Comprehensive Loss, Net of Tax	1,147	(2,695)	(22,258)	8,630	21,305	(1,062)	(23,806)	28,873
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	(115,663)	(116,810)	(114,115)	(76,381)	(85,011)	(106,316)	(115,663)	(76,381)
Retained Earnings [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shareholders' equity, beginning balance	2,138,532	1,985,250	1,878,116	1,854,257	1,729,319	1,467,024	1,878,116	1,467,024
Net income	80,466	214,888	157,646	71,193	178,992	314,132
Cash dividends	(61,654)	(61,606)	(58,722)	(54,116)	(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	$ 2,157,344	$ 2,138,532	$ 1,985,250	$ 1,871,334	$ 1,854,257	$ 1,729,319	$ 2,157,344	$ 1,871,334