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 Gain/(Loss) [Member]
Beginning Balance (in shares) at Sep. 30, 2012		65,452
Beginning Balance at Sep. 30, 2012	$ 1,939,422	$ 65,452	$ 160,540	$ 1,716,441	$ (3,011)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	350,224			350,224
Other comprehensive income (loss)	1,530				1,530
Restricted stock awards (in shares)		168
Vesting of restricted stock awards and performance unit awards		$ 168	(168)
Exercise of employee stock options (in shares)		437
Exercise of employee stock options	9,224	$ 437	8,787
Stock compensation expense	14,231		14,231
Repurchase and retirement of common shares (in shares)		(2,000)
Repurchase and retirement of common shares	(107,260)	$ (2,000)		(105,260)
Ending Balance (in shares) at Sep. 30, 2013		64,057
Ending 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)
Restricted stock awards (in shares)		180
Vesting of restricted stock awards and performance unit awards		$ 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)
Restricted stock 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)