CONSOLIDATED STATEMENTS OF CHANGES IN SHAREHOLDERS' EQUITY (USD $) In Thousands, unless otherwise specified	Total	Common Stock [Member]	Paid-In Capital [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Gain/(Loss) [Member]
Beginning Balance at Sep. 30, 2010	$ 1,370,134	$ 64,443	$ 133,095	$ 1,172,596	$ 0
Beginning Balance (in shares) at Sep. 30, 2010		64,443
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	271,674			271,674
Other comprehensive gain (loss)	(1,527)				(1,527)
Restricted stock awards		102	(102)
Restricted stock awards (in shares)		102
Exercise of employee stock options	6,192	415	5,777
Exercise of employee stock options (in shares)		415
Stock option and restricted stock award compensation expense	6,314		6,314
Ending Balance at Sep. 30, 2011	1,652,787	64,960	145,084	1,444,270	(1,527)
Ending Balance (in shares) at Sep. 30, 2011		64,960
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	272,171			272,171
Other comprehensive gain (loss)	(1,484)				(1,484)
Restricted stock awards		207	(207)
Restricted stock awards (in shares)		207
Exercise of employee stock options	5,546	285	5,261
Exercise of employee stock options (in shares)		285
Stock option and restricted stock award compensation expense	10,402		10,402
Ending Balance at Sep. 30, 2012	1,939,422	65,452	160,540	1,716,441	(3,011)
Ending Balance (in shares) at Sep. 30, 2012		65,452
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	350,224			350,224
Other comprehensive gain (loss)	1,530				1,530
Restricted stock awards		168	(168)
Restricted stock awards (in shares)		168
Exercise of employee stock options	9,224	437	8,787
Exercise of employee stock options (in shares)	437	437
Stock option and restricted stock award compensation expense	14,231		14,231
Repurchase and retirement of common shares	(107,260)	(2,000)		(105,260)
Repurchase and retirement of common shares (in shares)		(2,000)
Ending Balance at Sep. 30, 2013	$ 2,207,371	$ 64,057	$ 183,390	$ 1,961,405	$ (1,481)
Ending Balance (in shares) at Sep. 30, 2013		64,057