Condensed Consolidated Statements of Shareholders' Equity - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-In Capital	Retained Earnings	Accumulated Other Comprehensive Income (Loss)	Treasury Stock	Cumulative Effect, Period of Adoption, Adjustment	Cumulative Effect, Period of Adoption, Adjustment Retained Earnings	Cumulative Effect, Period of Adoption, Adjustment Accumulated Other Comprehensive Income (Loss)
Balance (in shares) at Sep. 30, 2018		112,009,000
Balance (in shares) at Sep. 30, 2018						3,015,000
Balance at Sep. 30, 2018	$ 4,382,735	$ 11,201	$ 500,393	$ 4,027,779	$ 16,550	$ (173,188)
Balance (Topic 606) at Sep. 30, 2018							$ (38)	$ (38)
Balance (ASU No. 2016-01) at Sep. 30, 2018							0	29,071	$ (29,071)
Increase (Decrease) in Shareholders' Equity
Net income (loss)	18,959			18,959
Other comprehensive income	225				225
Dividends declared	(78,488)			(78,488)
Exercise of employee stock options, net of shares withheld for employee taxes (in shares)						(125,000)
Exercise of employee stock options, net of shares withheld for employee taxes	224		(6,756)			$ 6,980
Vesting of restricted stock awards, net of shares withheld for employee taxes (in shares)		71,000				(215,000)
Vesting of restricted stock awards, net of shares withheld for employee taxes	(4,537)	$ 7	(16,673)			$ 12,129
Stock-based compensation	7,158		7,158
Balance (in shares) at Dec. 31, 2018		112,080,000
Balance (in shares) at Dec. 31, 2018						2,675,000
Balance at Dec. 31, 2018	4,326,238	$ 11,208	484,122	3,997,283	(12,296)	$ (154,079)
Balance (in shares) at Sep. 30, 2018		112,009,000
Balance (in shares) at Sep. 30, 2018						3,015,000
Balance at Sep. 30, 2018	4,382,735	$ 11,201	500,393	4,027,779	16,550	$ (173,188)
Balance (Topic 606) at Sep. 30, 2018							(38)	(38)
Balance (ASU No. 2016-01) at Sep. 30, 2018							$ 0	$ 29,071	$ (29,071)
Increase (Decrease) in Shareholders' Equity
Net income (loss)	(74,833)
Other comprehensive income	675
Balance (in shares) at Jun. 30, 2019		112,080,000
Balance (in shares) at Jun. 30, 2019						2,647,000
Balance at Jun. 30, 2019	4,095,054	$ 11,208	501,585	3,750,785	(16,085)	$ (152,439)
Balance (in shares) at Dec. 31, 2018		112,080,000
Balance (in shares) at Dec. 31, 2018						2,675,000
Balance at Dec. 31, 2018	4,326,238	$ 11,208	484,122	3,997,283	(12,296)	$ (154,079)
Increase (Decrease) in Shareholders' Equity
Net income (loss)	60,891			60,891
Other comprehensive income	224				224
Dividends declared	(78,466)			(78,466)
Exercise of employee stock options, net of shares withheld for employee taxes (in shares)						(7,000)
Exercise of employee stock options, net of shares withheld for employee taxes	302		(107)			$ 409
Vesting of restricted stock awards, net of shares withheld for employee taxes	0		(25)			$ 25
Stock-based compensation	9,431		9,431
Balance (in shares) at Mar. 31, 2019		112,080,000
Balance (in shares) at Mar. 31, 2019						2,668,000
Balance at Mar. 31, 2019	4,318,620	$ 11,208	493,421	3,979,708	(12,072)	$ (153,645)
Increase (Decrease) in Shareholders' Equity
Net income (loss)	(154,683)			(154,683)
Other comprehensive income	226				226
Dividends declared	(78,479)			(78,479)
Exercise of employee stock options, net of shares withheld for employee taxes (in shares)						(15,000)
Exercise of employee stock options, net of shares withheld for employee taxes	645		(225)			$ 870
Vesting of restricted stock awards, net of shares withheld for employee taxes (in shares)						(6,000)
Vesting of restricted stock awards, net of shares withheld for employee taxes	(153)		(489)			$ 336
Stock-based compensation	8,878		8,878
Reclassification of stranded tax effect for adoption of ASU No. 2018-02	0			4,239	(4,239)
Balance (in shares) at Jun. 30, 2019		112,080,000
Balance (in shares) at Jun. 30, 2019						2,647,000
Balance at Jun. 30, 2019	$ 4,095,054	$ 11,208	501,585	3,750,785	(16,085)	$ (152,439)
Balance (in shares) at Sep. 30, 2019	112,080,262	112,080,000
Balance (in shares) at Sep. 30, 2019	3,642,358					3,642,000
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)						(110,000)
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)		71,000				(258,000)
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		112,151,000
Balance (in shares) at Dec. 31, 2019						3,274,000
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	112,080,262	112,080,000
Balance (in shares) at Sep. 30, 2019	3,642,358					3,642,000
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)	(435,534)
Other comprehensive income	$ 1,553
Balance (in shares) at Jun. 30, 2020	112,151,563	112,151,000
Balance (in shares) at Jun. 30, 2020	4,680,268					4,680,000
Balance at Jun. 30, 2020	$ 3,396,057	$ 11,215	514,673	3,096,168	(27,082)	$ (198,917)
Balance (in shares) at Dec. 31, 2019		112,151,000
Balance (in shares) at Dec. 31, 2019						3,274,000
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)						(1,000)
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)						1,460,000
Share repurchases	(28,504)					$ (28,504)
Balance (in shares) at Mar. 31, 2020		112,151,000
Balance (in shares) at Mar. 31, 2020						4,733,000
Balance at Mar. 31, 2020	3,461,006	$ 11,215	509,928	3,168,966	(27,603)	$ (201,500)
Increase (Decrease) in Shareholders' Equity
Net income (loss)	(45,599)			(45,599)
Other comprehensive income	521				521
Dividends declared	(27,199)			(27,199)
Exercise of employee stock options, net of shares withheld for employee taxes	0		0			$ 0
Vesting of restricted stock awards, net of shares withheld for employee taxes (in shares)						(53,000)
Vesting of restricted stock awards, net of shares withheld for employee taxes	(296)		(2,879)			$ 2,583
Stock-based compensation	$ 7,624		7,624
Balance (in shares) at Jun. 30, 2020	112,151,563	112,151,000
Balance (in shares) at Jun. 30, 2020	4,680,268					4,680,000
Balance at Jun. 30, 2020	$ 3,396,057	$ 11,215	$ 514,673	$ 3,096,168	$ (27,082)	$ (198,917)