Shareholders' Equity - Components of Equity (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Common stock outstanding, beginning balance (in shares)	125,882,477
Shareholders' equity, beginning balance	$ 6,791,203	$ 5,750,223
Net income	217,678	178,673
Other comprehensive income (loss)	60,121	1,052
Cash dividends	(79,023)	(69,557)
Public and other stock offerings	220,009	263,272
Stock-based compensation plans	$ 3,168	4,112
Common stock outstanding, ending balance (in shares)	128,152,961
Shareholders' equity, ending balance	$ 7,213,156	$ 6,127,775
Cash dividends per share (USD per share)	$ 0.625	$ 0.575
Common stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Common stock outstanding, beginning balance (in shares)	125,882,477	119,338,925
Shareholders' equity, beginning balance	$ 629	$ 597
Public and other stock offerings (in shares)	2,126,118	2,758,929
Public and other stock offerings	$ 11	$ 13
Stock-based compensation plans (in shares)	144,366	164,549
Stock-based compensation plans	$ 1	$ 1
Common stock outstanding, ending balance (in shares)	128,152,961	122,262,403
Shareholders' equity, ending balance	$ 641	$ 611
Additional Paid-in Capital
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shareholders' equity, beginning balance	4,377,149	3,712,194
Public and other stock offerings	219,998	263,259
Stock-based compensation plans	3,167	4,111
Shareholders' equity, ending balance	4,600,314	3,979,564
Accumulated Other Comprehensive Income (Loss)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shareholders' equity, beginning balance	(57,589)	(114,583)
Other comprehensive income (loss)	60,121	1,052
Shareholders' equity, ending balance	2,532	(113,531)
Retained Earnings
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shareholders' equity, beginning balance	2,471,014	2,152,015
Net income	217,678	178,673
Cash dividends	(79,023)	(69,557)
Shareholders' equity, ending balance	$ 2,609,669	$ 2,261,131