Condensed Consolidated Statements of Shareholders' Equity - USD ($) shares in Thousands, $ in Thousands	Total	Cumulative Effect, Period of Adoption, Adjustment	Common Stock	Additional Paid-In Capital	Retained Earnings	Retained Earnings Cumulative Effect, Period of Adoption, Adjustment	Accumulated Other Comprehensive Income (Loss)	Treasury Stock
Balance (in shares) at Sep. 30, 2019			112,080					3,642
Beginning balance at Sep. 30, 2019	$ 4,012,223		$ 11,208	$ 510,305	$ 3,714,307		$ (28,635)	$ (194,962)
Increase (Decrease) in Shareholders' Equity
Net income (loss)	30,605				30,605
Other comprehensive income	516						516
Dividends declared	(78,652)				(78,652)
Exercise of employee stock options, net of shares withheld for employee taxes (in shares)								(110)
Exercise of employee stock options, net of shares withheld for employee taxes	4,045			(3,103)				$ 7,148
Vesting of restricted stock awards, net of shares withheld for employee taxes (in shares)			71					(258)
Vesting of restricted stock awards, net of shares withheld for employee taxes	(3,401)		$ 7	(18,126)				$ 14,718
Stock-based compensation	10,201			10,201
Balance (in shares) at Dec. 31, 2019			112,151					3,274
Ending balance at Dec. 31, 2019	3,975,537		$ 11,215	499,277	3,666,260		(28,119)	$ (173,096)
Balance (in shares) at Sep. 30, 2019			112,080					3,642
Beginning balance at Sep. 30, 2019	4,012,223		$ 11,208	510,305	3,714,307		(28,635)	$ (194,962)
Increase (Decrease) in Shareholders' Equity
Net income (loss)	(389,935)
Other comprehensive income	1,032
Balance (in shares) at Mar. 31, 2020			112,151					4,733
Ending balance at Mar. 31, 2020	3,461,006		$ 11,215	509,928	3,168,966		(27,603)	$ (201,500)
Balance (in shares) at Sep. 30, 2019			112,080					3,642
Beginning balance at Sep. 30, 2019	4,012,223		$ 11,208	510,305	3,714,307		(28,635)	$ (194,962)
Balance (in shares) at Sep. 30, 2020			112,151					4,663
Ending balance at Sep. 30, 2020	$ 3,318,514	$ (1,251)	$ 11,215	521,628	3,010,012	$ (1,251)	(26,188)	$ (198,153)
Increase (Decrease) in Shareholders' Equity
Accounting Standards Update [Extensible List]	us-gaap:AccountingStandardsUpdate201613Member
Balance (in shares) at Dec. 31, 2019			112,151					3,274
Beginning balance at Dec. 31, 2019	$ 3,975,537		$ 11,215	499,277	3,666,260		(28,119)	$ (173,096)
Increase (Decrease) in Shareholders' Equity
Net income (loss)	(420,540)				(420,540)
Other comprehensive income	516						516
Dividends declared	(76,754)				(76,754)
Exercise of employee stock options, net of shares withheld for employee taxes	0			(47)				$ 47
Vesting of restricted stock awards, net of shares withheld for employee taxes (in shares)								(1)
Vesting of restricted stock awards, net of shares withheld for employee taxes	0			(53)				$ 53
Stock-based compensation	10,751			10,751
Share repurchases (in shares)								1,460
Share repurchases	(28,504)							$ (28,504)
Balance (in shares) at Mar. 31, 2020			112,151					4,733
Ending balance at Mar. 31, 2020	3,461,006		$ 11,215	509,928	3,168,966		(27,603)	$ (201,500)
Balance (in shares) at Sep. 30, 2020			112,151					4,663
Beginning balance at Sep. 30, 2020	3,318,514	(1,251)	$ 11,215	521,628	3,010,012	(1,251)	(26,188)	$ (198,153)
Increase (Decrease) in Shareholders' Equity
Net income (loss)	(70,431)				(70,431)
Other comprehensive income	457						457
Dividends declared	(27,324)				(27,324)
Vesting of restricted stock awards, net of shares withheld for employee taxes (in shares)			72					(295)
Vesting of restricted stock awards, net of shares withheld for employee taxes	(2,117)		$ 7	(16,742)				$ 14,618
Stock-based compensation	7,451			7,451
Other	(381)			(381)
Balance (in shares) at Dec. 31, 2020			112,223					4,368
Ending balance at Dec. 31, 2020	3,224,918		$ 11,222	511,956	2,911,006		(25,731)	$ (183,535)
Balance (in shares) at Sep. 30, 2020			112,151					4,663
Beginning balance at Sep. 30, 2020	3,318,514	$ (1,251)	$ 11,215	521,628	3,010,012	$ (1,251)	(26,188)	$ (198,153)
Increase (Decrease) in Shareholders' Equity
Net income (loss)	(191,434)
Other comprehensive income	914
Balance (in shares) at Mar. 31, 2021			112,223					4,329
Ending balance at Mar. 31, 2021	3,083,696		$ 11,222	516,870	2,762,735		(25,274)	$ (181,857)
Balance (in shares) at Dec. 31, 2020			112,223					4,368
Beginning balance at Dec. 31, 2020	3,224,918		$ 11,222	511,956	2,911,006		(25,731)	$ (183,535)
Increase (Decrease) in Shareholders' Equity
Net income (loss)	(121,003)				(121,003)
Other comprehensive income	457						457
Dividends declared	(27,268)				(27,268)
Vesting of restricted stock awards, net of shares withheld for employee taxes (in shares)								(39)
Vesting of restricted stock awards, net of shares withheld for employee taxes	0			(1,678)				$ 1,678
Stock-based compensation	6,826			6,826
Other	(234)			(234)
Balance (in shares) at Mar. 31, 2021			112,223					4,329
Ending balance at Mar. 31, 2021	$ 3,083,696		$ 11,222	$ 516,870	$ 2,762,735		$ (25,274)	$ (181,857)