Consolidated Statements of Shareholders' Equity - USD ($) $ in Thousands	Common Stock	Additional Paid-In Capital	Retained Earnings	Accumulated Other Comprehensive (Loss) Income	Treasury Stock	Total
Balance at beginning of period at Sep. 30, 2015	$ 11,099	$ 420,141	$ 4,648,346	$ (1,377)	$ (182,363)	$ 4,895,846
Balance (in shares) at Sep. 30, 2015	110,987,000
Balance (in shares) at Sep. 30, 2015					3,220,000
Increase (Decrease) in Shareholders' Equity
Net income (loss)			(56,828)			(56,828)
Other comprehensive income (loss)				1,173		1,173
Dividends declared			(301,711)			(301,711)
Exercise of employee stock options, net of shares withheld for employee taxes	$ 22	6,937			$ (5,919)	1,040
Exercise of employee stock options, net of shares withheld for employee taxes (in shares)	220,000				99,000
Tax benefit of stock-based awards		934				934
Vesting of restricted stock awards, net of shares withheld for employee taxes	$ 19	(3,943)			$ 12	(3,912)
Vesting of restricted stock awards, net of shares withheld for employee taxes (in shares)	193,000				3,000
Stock-based compensation		24,383				24,383
Balance at end of period at Sep. 30, 2016	$ 11,140	448,452	4,289,807	(204)	$ (188,270)	4,560,925
Balance (in shares) at Sep. 30, 2016	111,400,000
Balance (in shares) at Sep. 30, 2016					3,322,000
Increase (Decrease) in Shareholders' Equity
Net income (loss)			(128,212)			(128,212)
Other comprehensive income (loss)				2,504		2,504
Dividends declared			(305,909)			(305,909)
Exercise of employee stock options, net of shares withheld for employee taxes	$ 42	15,738			$ (5,246)	10,534
Exercise of employee stock options, net of shares withheld for employee taxes (in shares)	415,000				88,000
Tax benefit of stock-based awards		4,414				4,414
Vesting of restricted stock awards, net of shares withheld for employee taxes	$ 14	(7,539)			$ 1,677	(5,848)
Vesting of restricted stock awards, net of shares withheld for employee taxes (in shares)	142,000				(57,000)
Stock-based compensation		26,183				26,183
Balance at end of period at Sep. 30, 2017	$ 11,196	487,248	3,855,686	2,300	$ (191,839)	$ 4,164,591
Balance (in shares) at Sep. 30, 2017	111,957,000					111,956,875
Balance (in shares) at Sep. 30, 2017					3,353,000	3,352,828
Increase (Decrease) in Shareholders' Equity
Net income (loss)			482,672			$ 482,672
Other comprehensive income (loss)				14,250		14,250
Dividends declared			(310,024)			(310,024)
Exercise of employee stock options, net of shares withheld for employee taxes		(7,557)			$ 10,992	3,435
Exercise of employee stock options, net of shares withheld for employee taxes (in shares)	1,000				(202,000)
Adoption of ASU 2016-09 (Note 2) | ASU 2016-09		872	(555)			317
Vesting of restricted stock awards, net of shares withheld for employee taxes	$ 5	(11,857)			$ 7,659	(4,193)
Vesting of restricted stock awards, net of shares withheld for employee taxes (in shares)	51,000				(136,000)
Stock-based compensation		31,687				31,687
Balance at end of period at Sep. 30, 2018	$ 11,201	$ 500,393	$ 4,027,779	$ 16,550	$ (173,188)	$ 4,382,735
Balance (in shares) at Sep. 30, 2018	112,009,000					112,008,961
Balance (in shares) at Sep. 30, 2018					3,015,000	3,015,243