CONSOLIDATED STATEMENT OF SHAREHOLDERS' EQUITY - USD ($) $ in Thousands	Total	Common Stock [Member]	Additional Paid In Capital [Member]	Accumulated Other Comprehensive Income [Member]	Retained Earnings [Member]
Common stock shares outstanding, balance at Sep. 30, 2014		100,388,092
Shareholders' equity, beginning balance at Sep. 30, 2014	$ 3,086,232	$ 502	$ 2,180,151	$ (12,393)	$ 917,972
Net income	315,075				315,075
Other comprehensive income (loss)	(96,937)			(96,937)
Repurchase of equity awards, shares		(148,464)
Repurchase of equity awards, amount	(7,985)	$ (1)	(7,984)
Cash dividends	(160,018)				(160,018)
Common stock issued:
Direct stock purchase plan, shares		176,391
Direct stock purchase plan, amount	10,626	$ 1	10,625
Retirement savings plan, shares		398,047
Retirement savings plan, amount	20,326	$ 2	20,324
1998 Long-term incentive plan, shares		664,752
1998 Long-term incentive plan, amount	2,266	$ 3	2,263
Employee stock-based compensation	25,212		25,212
Common stock shares outstanding, ending balance at Sep. 30, 2015		101,478,818
Shareholders' equity, ending balance at Sep. 30, 2015	3,194,797	$ 507	2,230,591	(109,330)	1,073,029
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, shares		1,360,756
Public offering, amount	98,574	$ 7	98,567
Direct stock purchase plan, shares		133,133
Direct stock purchase plan, amount	9,229	$ 1	9,228
Retirement savings plan, shares		359,414
Retirement savings plan, amount	25,049	$ 2	25,047
1998 Long-term incentive plan, shares		598,439
1998 Long-term incentive plan, amount	3,178	$ 3	3,175
Employee stock-based compensation	21,419		21,419
Common stock shares outstanding, ending balance 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
Cumulative Effect of New Accounting Principle in Period of Adoption	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, shares		1,303,494
Public offering, amount	98,755	$ 6	98,749
Direct stock purchase plan, shares		112,592
Direct stock purchase plan, amount	8,971	$ 1	8,970
Retirement savings plan, shares		228,326
Retirement savings plan, amount	17,552	$ 1	17,551
1998 Long-term incentive plan, shares		529,662
1998 Long-term incentive plan, amount	3,701	$ 3	3,698
Employee stock-based compensation	19,370		19,370
Common stock shares outstanding, ending balance at Sep. 30, 2017		106,104,634
Shareholders' equity, ending balance at Sep. 30, 2017	$ 3,898,666	$ 531	$ 2,536,365	$ (105,254)	$ 1,467,024