CONSOLIDATED STATEMENT OF SHAREHOLDERS' EQUITY - USD ($) $ in Thousands	Total	Common stock	Additional Paid-in Capital	Accumulated Other Comprehensive Income (Loss)	Retained Earnings	Cumulative effect of accounting change Accumulated Other Comprehensive Income (Loss)	Cumulative effect of accounting change Retained Earnings
Common stock shares outstanding, balance (in shares) at Sep. 30, 2018		111,273,683
Shareholders' equity, beginning balance at Sep. 30, 2018	$ 4,769,951	$ 556	$ 2,974,926	$ (83,647)	$ 1,878,116	$ (8,210)	$ 8,210
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income	511,406				511,406
Other comprehensive income (loss)	(22,726)			(22,726)
Cash dividends	(245,717)				(245,717)
Common stock issued:
Public offering (in shares)		7,574,111
Public offering	694,103	$ 38	694,065
Direct stock purchase plan (in shares)		110,063
Direct stock purchase plan	11,071	$ 1	11,070
Retirement savings plan (in shares)		81,456
Retirement savings plan	8,252		8,252
1998 Long-term incentive plan (in shares)		299,612
1998 Long-term incentive plan	2,948	$ 2	2,946
Employee stock-based compensation	20,935		20,935
Common stock shares outstanding, ending balance (in shares) at Sep. 30, 2019		119,338,925
Shareholders' equity, ending balance at Sep. 30, 2019	5,750,223	$ 597	3,712,194	(114,583)	2,152,015
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income	601,443				601,443
Other comprehensive income (loss)	56,994			56,994
Cash dividends	(282,444)				(282,444)
Common stock issued:
Public offering (in shares)		6,101,916
Public offering	624,302	$ 30	624,272
Direct stock purchase plan (in shares)		107,989
Direct stock purchase plan	11,326	$ 1	11,325
Retirement savings plan (in shares)		78,941
Retirement savings plan	8,222		8,222
1998 Long-term incentive plan (in shares)		254,706
1998 Long-term incentive plan	2,749	$ 1	2,748
Employee stock-based compensation	$ 18,388		18,388
Common stock shares outstanding, ending balance (in shares) at Sep. 30, 2020	125,882,477	125,882,477
Shareholders' equity, ending 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 (loss)	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	$ 0	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	132,419,754
Shareholders' equity, ending balance at Sep. 30, 2021	$ 7,906,889	$ 662	$ 5,023,751	$ 69,803	$ 2,812,673