CONSOLIDATED STATEMENTS OF SHAREHOLDERS' EQUITY - USD ($) $ in Thousands	Total	Common stock	Additional Paid-in Capital	Accumulated Other Comprehensive Income (Loss)	Retained Earnings
Common stock shares outstanding, balance (in shares) at Sep. 30, 2020		125,882,477
Shareholders' equity, beginning balance at Sep. 30, 2020	$ 6,791,203	$ 629	$ 4,377,149	$ (57,589)	$ 2,471,014
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	665,563				665,563
Other comprehensive income	127,392			127,392
Cash dividends	(323,904)				(323,904)
Common stock issued:
Public offering (in shares)		6,130,875
Public offering	606,667	$ 31	606,636
Direct stock purchase plan (in shares)		79,921
Direct stock purchase plan	7,715		7,715
Retirement savings plan (in shares)		84,265
Retirement savings plan	8,126	$ 1	8,125
1998 Long-term incentive plan (in shares)		242,216
1998 Long-term incentive plan	3,092	$ 1	3,091
Employee stock-based compensation	21,035		21,035
Common stock shares outstanding, ending balance (in shares) at Sep. 30, 2021		132,419,754
Shareholders' equity, ending balance at Sep. 30, 2021	7,906,889	$ 662	5,023,751	69,803	2,812,673
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	774,398				774,398
Other comprehensive income	299,309			299,309
Cash dividends	(375,914)				(375,914)
Common stock issued:
Public offering (in shares)		7,907,883
Public offering	776,805	$ 40	776,765
Direct stock purchase plan (in shares)		68,693
Direct stock purchase plan	7,495		7,495
Retirement savings plan (in shares)		72,339
Retirement savings plan	7,908		7,908
1998 Long-term incentive plan (in shares)		427,929
1998 Long-term incentive plan	2,398	$ 2	2,396
Employee stock-based compensation	$ 19,803		19,803
Common stock shares outstanding, ending balance (in shares) at Sep. 30, 2022	140,896,598	140,896,598
Shareholders' equity, ending balance at Sep. 30, 2022	$ 9,419,091	$ 704	5,838,118	369,112	3,211,157
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	885,862				885,862
Other comprehensive income	149,416			149,416
Cash dividends	(430,345)				(430,345)
Common stock issued:
Public offering (in shares)		7,272,261
Public offering	806,949	$ 36	806,913
Direct stock purchase plan (in shares)		64,871
Direct stock purchase plan	7,429		7,429
Retirement savings plan (in shares)		69,716
Retirement savings plan	7,966	$ 1	7,965
1998 Long-term incentive plan (in shares)		189,337
1998 Long-term incentive plan	2,108	$ 1	2,107
Employee stock-based compensation	$ 21,588		21,588
Common stock shares outstanding, ending balance (in shares) at Sep. 30, 2023	148,492,783	148,492,783
Shareholders' equity, ending balance at Sep. 30, 2023	$ 10,870,064	$ 742	$ 6,684,120	$ 518,528	$ 3,666,674