Shareholders' Equity - Components of Equity (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended						9 Months Ended
	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Jun. 30, 2020	Jun. 30, 2019
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Common stock outstanding (in shares)			119,338,925				119,338,925
Shareholders' equity, beginning balance	$ 6,304,415	$ 6,127,775	$ 5,750,223	$ 5,508,101	$ 5,348,195	$ 4,769,951	$ 5,750,223	$ 4,769,951
Net income	117,791	239,646	178,673	80,466	214,888	157,646	536,110	453,000
Other comprehensive income (loss)	(4,086)	890	1,052	1,147	(2,695)	(22,258)	$ (2,144)	(23,806)
Cash dividends	(70,597)	(70,520)	(69,557)	(61,654)	(61,606)	(58,722)
Public and other stock offerings	105,804	3,096	263,272	103,430	5,454	498,975
Stock-based compensation plans	$ 8,144	3,528	4,112	10,506	3,865	2,603
Common stock outstanding (in shares)	123,351,267						123,351,267
Shareholders' equity, ending balance	$ 6,461,471	$ 6,304,415	$ 6,127,775	$ 5,641,996	$ 5,508,101	$ 5,348,195	$ 6,461,471	$ 5,641,996
Cash dividends per share (USD per share)	$ 0.575	$ 0.575	$ 0.575	$ 0.525	$ 0.525	$ 0.525	$ 1.725	$ 1.575
Common stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Common stock outstanding (in shares)	122,308,725	122,262,403	119,338,925	116,982,903	116,892,959	111,273,683	119,338,925	111,273,683
Shareholders' equity, beginning balance	$ 612	$ 611	$ 597	$ 585	$ 584	$ 556	$ 597	$ 556
Public and other stock offerings (in shares)	965,576	38,662	2,758,929	1,127,244	61,006	5,434,812
Public and other stock offerings	$ 5	$ 1	$ 13	$ 5	$ 1	$ 27
Stock-based compensation plans (in shares)	76,966	7,660	164,549	85,966	28,938	184,464
Stock-based compensation plans	$ 0	$ 0	$ 1	$ 1	$ 0	$ 1
Common stock outstanding (in shares)	123,351,267	122,308,725	122,262,403	118,196,113	116,982,903	116,892,959	123,351,267	118,196,113
Shareholders' equity, ending balance	$ 617	$ 612	$ 611	$ 591	$ 585	$ 584	$ 617	$ 591
Additional Paid-in Capital
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shareholders' equity, beginning balance	3,986,187	3,979,564	3,712,194	3,485,794	3,476,476	2,974,926	3,712,194	2,974,926
Public and other stock offerings	105,799	3,095	263,259	103,425	5,453	498,948
Stock-based compensation plans	8,144	3,528	4,111	10,505	3,865	2,602
Shareholders' equity, ending balance	4,100,130	3,986,187	3,979,564	3,599,724	3,485,794	3,476,476	4,100,130	3,599,724
Accumulated Other Comprehensive Income (Loss)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shareholders' equity, beginning balance	(112,641)	(113,531)	(114,583)	(116,810)	(114,115)	(83,647)	(114,583)	(83,647)
Other comprehensive income (loss)	(4,086)	890	1,052	1,147	(2,695)	(22,258)
Shareholders' equity, ending balance	(116,727)	(112,641)	(113,531)	(115,663)	(116,810)	(114,115)	(116,727)	(115,663)
Retained Earnings
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shareholders' equity, beginning balance	2,430,257	2,261,131	2,152,015	2,138,532	1,985,250	1,878,116	2,152,015	1,878,116
Net income	117,791	239,646	178,673	80,466	214,888	157,646
Cash dividends	(70,597)	(70,520)	(69,557)	(61,654)	(61,606)	(58,722)
Shareholders' equity, ending balance	$ 2,477,451	$ 2,430,257	$ 2,261,131	$ 2,157,344	$ 2,138,532	1,985,250	$ 2,477,451	2,157,344
Cumulative effect of accounting change
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shareholders' equity, beginning balance						(8,210)		(8,210)
Cumulative effect of accounting change | Accumulated Other Comprehensive Income (Loss)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shareholders' equity, beginning balance						(8,210)		(8,210)
Cumulative effect of accounting change | Retained Earnings
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shareholders' equity, beginning balance						$ 8,210		$ 8,210