CONSOLIDATED STATEMENTS OF CHANGES IN SHAREHOLDERS' EQUITY - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock [Member]	Paid-In Capital [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Income/(Loss) [Member]
Beginning Balance (in shares) at Sep. 30, 2013		64,057
Beginning Balance at Sep. 30, 2013	$ 2,207,371	$ 64,057	$ 183,390	$ 1,961,405	$ (1,481)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	340,822			340,822
Other comprehensive income (loss)	5,042				5,042
Dividends	(16,090)			(16,090)
Vesting of restricted stock awards and performance unit awards (in shares)		180
Vesting of restricted stock awards and performance unit awards	0	$ 180	(180)
Exercise of employee stock options (in shares)		125
Exercise of employee stock options	3,688	$ 125	3,563
Stock compensation expense	14,691		14,691
Ending Balance (in shares) at Sep. 30, 2014		64,362
Ending Balance at Sep. 30, 2014	2,555,524	$ 64,362	201,464	2,286,137	3,561
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	432,573			432,573
Other comprehensive income (loss)	(4,289)				(4,289)
Dividends	(48,562)			(48,562)
Vesting of restricted stock awards and performance unit awards (in shares)		224
Vesting of restricted stock awards and performance unit awards	(1,968)	$ 224	(2,192)
Exercise of employee stock options (in shares)		68
Exercise of employee stock options	1,064	$ 68	996
Stock compensation expense	12,828		12,828
Ending Balance (in shares) at Sep. 30, 2015		64,654
Ending Balance at Sep. 30, 2015	2,947,170	$ 64,654	213,096	2,670,148	(728)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	265,272			265,272
Other comprehensive income (loss)	(1,066)				(1,066)
Dividends	(5,581)			(5,581)
Vesting of restricted stock awards and performance unit awards (in shares)		145
Vesting of restricted stock awards and performance unit awards	(465)	$ 145	(610)
Stock compensation expense	11,862		11,862
Stock compensation windfall tax benefits	13,194		13,194
Ending Balance (in shares) at Sep. 30, 2016		64,799
Ending Balance at Sep. 30, 2016	$ 3,230,386	$ 64,799	$ 237,542	$ 2,929,839	$ (1,794)