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, 2017		106,104,634
Shareholders' equity, beginning balance at Sep. 30, 2017	$ 3,898,666	$ 531	$ 2,536,365	$ (105,254)	$ 1,467,024	$ (22,934)	$ 22,934
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income	603,064				603,064
Other comprehensive income (loss)	44,541			44,541
Cash dividends	(214,906)				(214,906)
Common stock issued:
Public offering (in shares)		4,558,404
Public offering	395,092	$ 22	395,070
Direct stock purchase plan (in shares)		131,213
Direct stock purchase plan	11,323	$ 1	11,322
Retirement savings plan (in shares)		94,081
Retirement savings plan	8,240	$ 0	8,240
1998 Long-term incentive plan (in shares)		385,351
1998 Long-term incentive plan	3,471	$ 2	3,469
Employee stock-based compensation	20,460		20,460
Common stock shares outstanding, ending balance (in shares) at Sep. 30, 2018		111,273,683
Shareholders' equity, ending 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	$ 0	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	119,338,925
Shareholders' equity, ending balance at Sep. 30, 2019	$ 5,750,223	$ 597	3,712,194	(114,583)	2,152,015	$ (8,210)
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