Consolidated Statements of Shareholders' Equity - USD ($) $ in Thousands	Total	Cumulative Effect, Period of Adoption, Adjustment	Common Stock	Additional Paid-In Capital	Additional Paid-In Capital Cumulative Effect, Period of Adoption, Adjustment	Retained Earnings	Retained Earnings Cumulative Effect, Period of Adoption, Adjustment	Accumulated Other Comprehensive Income (Loss)	Accumulated Other Comprehensive Income (Loss) Cumulative Effect, Period of Adoption, Adjustment	Treasury Stock
Balance (in shares) at Sep. 30, 2017			111,957,000							3,353,000
Balance at Sep. 30, 2017	$ 4,164,591	$ 317	$ 11,196	$ 487,248	$ 872	$ 3,855,686	$ (555)	$ 2,300		$ (191,839)
Comprehensive income (loss):
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 (in shares)			1,000							(202,000)
Exercise of employee stock options, net of shares withheld for employee taxes	3,435		$ 0	(7,557)						$ 10,992
Vesting of restricted stock awards, net of shares withheld for employee taxes (in shares)			51,000							(136,000)
Vesting of restricted stock awards, net of shares withheld for employee taxes	(4,193)		$ 5	(11,857)						$ 7,659
Stock-based compensation	31,687			31,687
Balance (in shares) at Sep. 30, 2018			112,009,000							3,015,000
Balance at Sep. 30, 2018	4,382,735		$ 11,201	500,393		4,027,779		16,550		$ (173,188)
Balance (ASU 2014-09) at Sep. 30, 2018		(38)					(38)
Balance (ASU 2016-01) at Sep. 30, 2018		$ 0					$ 29,071		$ (29,071)
Comprehensive income (loss):
Net income (loss)	(33,656)					(33,656)
Other comprehensive income (loss)	(11,875)							(11,875)
Dividends declared	(313,088)					(313,088)
Exercise of employee stock options, net of shares withheld for employee taxes (in shares)										(151,000)
Exercise of employee stock options, net of shares withheld for employee taxes	1,321			(7,153)						$ 8,474
Vesting of restricted stock awards, net of shares withheld for employee taxes (in shares)			71,000							(222,000)
Vesting of restricted stock awards, net of shares withheld for employee taxes	(4,689)		$ 7	(17,227)						$ 12,531
Stock-based compensation	34,292			34,292
Share repurchases (in shares)										1,000,000
Share repurchases	(42,779)									$ (42,779)
Reclassification of stranded tax effect for adoption of ASU No. 2018-02	$ 0					4,239		(4,239)
Balance (in shares) at Sep. 30, 2019	112,080,262		112,080,000							3,642,000
Balance at Sep. 30, 2019	$ 4,012,223		$ 11,208	510,305		3,714,307		(28,635)		$ (194,962)
Comprehensive income (loss):
Net income (loss)	(494,497)					(494,497)
Other comprehensive income (loss)	2,447							2,447
Dividends declared	(209,798)					(209,798)
Exercise of employee stock options, net of shares withheld for employee taxes (in shares)										(110,000)
Exercise of employee stock options, net of shares withheld for employee taxes	4,044			(3,151)						$ 7,195
Vesting of restricted stock awards, net of shares withheld for employee taxes (in shares)			71,000							(329,000)
Vesting of restricted stock awards, net of shares withheld for employee taxes	(3,729)		$ 7	(21,855)						$ 18,119
Stock-based compensation	36,329			36,329
Share repurchases (in shares)										1,460,000
Share repurchases	$ (28,505)									$ (28,505)
Balance (in shares) at Sep. 30, 2020	112,151,563		112,151,000							4,663,000
Balance at Sep. 30, 2020	$ 3,318,514		$ 11,215	$ 521,628		$ 3,010,012		$ (26,188)		$ (198,153)