CONSOLIDATED STATEMENT 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, 2015		101,478,818
Shareholders' equity, beginning balance at Sep. 30, 2015	$ 3,194,797	$ 507	$ 2,230,591	$ (109,330)	$ 1,073,029
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	350,104				350,104
Other comprehensive income (loss)	(78,692)			(78,692)
Cash dividends	(175,126)				(175,126)
Common stock issued:
Public offering (in shares)		1,360,756
Public offering	98,574	$ 7	98,567
Direct stock purchase plan (in shares)		133,133
Direct stock purchase plan	9,229	$ 1	9,228
Retirement savings plan (in shares)		359,414
Retirement savings plan	25,049	$ 2	25,047
1998 Long-term incentive plan (in shares)		598,439
1998 Long-term incentive plan	3,178	$ 3	3,175
Employee stock-based compensation	21,419		21,419
Common stock shares outstanding, ending balance (in shares) at Sep. 30, 2016		103,930,560
Shareholders' equity, ending balance at Sep. 30, 2016	3,463,059	$ 520	2,388,027	(188,022)	1,262,534
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative effect of accounting change	14,527				14,527
Net income	396,421				396,421
Other comprehensive income (loss)	82,768			82,768
Cash dividends	(191,931)				(191,931)
Common stock issued:
Public offering (in shares)		1,303,494
Public offering	98,755	$ 6	98,749
Direct stock purchase plan (in shares)		112,592
Direct stock purchase plan	8,971	$ 1	8,970
Retirement savings plan (in shares)		228,326
Retirement savings plan	17,552	$ 1	17,551
1998 Long-term incentive plan (in shares)		529,662
1998 Long-term incentive plan	3,701	$ 3	3,698
Employee stock-based compensation	$ 19,370		19,370
Common stock shares outstanding, ending balance (in shares) at Sep. 30, 2017	106,104,634	106,104,634
Shareholders' equity, ending balance at Sep. 30, 2017	$ 3,898,666	$ 531	2,536,365	(105,254)	1,467,024
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		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	111,273,683
Shareholders' equity, ending balance at Sep. 30, 2018	$ 4,769,951	$ 556	$ 2,974,926	(83,647)	1,878,116
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative effect of accounting change	$ 22,900			$ (22,934)	$ 22,934