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, 2011	$ 1,652,787	$ 64,960	$ 145,084	$ 1,444,270	$ (1,527)
Beginning Balance (in shares) at Sep. 30, 2011		64,960
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	272,171			272,171
Other comprehensive income (loss)	(1,484)				(1,484)
Vesting of 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 income (loss)	1,530				1,530
Vesting of 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
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
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	340,822			340,822
Other comprehensive income (loss)	5,042				5,042
Dividends declared	(16,090)			(16,090)
Vesting of restricted stock awards		180	(180)
Restricted stock awards (in shares)		180
Exercise of employee stock options	3,688	125	3,563
Exercise of employee stock options (in shares)		125
Stock option and restricted stock award compensation expense	14,691		14,691
Ending Balance at Sep. 30, 2014	$ 2,555,524	$ 64,362	$ 201,464	$ 2,286,137	$ 3,561
Ending Balance (in shares) at Sep. 30, 2014		64,362