Condensed Consolidated Statement of Shareholders' Equity - USD ($) $ in Thousands	Common Stock	Additional Paid-In Capital	Retained Earnings	Accumulated Other Comprehensive Income	Treasury Stock	Total
Balance at beginning 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
Balance (in shares) at Sep. 30, 2017					3,353,000
Increase (Decrease) in Shareholders' Equity
Net income (loss)			500,106			500,106
Other comprehensive income (loss)				(261)		(261)
Dividends declared			(76,911)			(76,911)
Exercise of employee stock options, net of shares withheld for employee taxes		(3,976)			$ 3,485	(491)
Exercise of employee stock options, net of shares withheld for employee taxes (in shares)	1,000				(61,000)
Vesting of restricted stock awards, net of shares withheld for employee taxes	$ 5	(11,317)			$ 7,224	(4,088)
Vesting of restricted stock awards, net of shares withheld for employee taxes (in shares)	51,000				(128,000)
Stock-based compensation		7,087				7,087
Cumulate effect adjustment for adoption | ASU 2016-09		872	(555)			317
Balance at end of period at Dec. 31, 2017	$ 11,201	479,914	4,278,326	2,039	$ (181,130)	4,590,350
Balance (in shares) at Dec. 31, 2017	112,009,000
Balance (in shares) at Dec. 31, 2017					3,164,000
Balance at beginning 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
Balance (in shares) at Sep. 30, 2017					3,353,000
Increase (Decrease) in Shareholders' Equity
Net income (loss)						488,227
Other comprehensive income (loss)						(7,521)
Balance at end of period at Mar. 31, 2018	$ 11,201	487,135	4,189,497	(5,221)	$ (179,459)	4,503,153
Balance (in shares) at Mar. 31, 2018	112,009,000
Balance (in shares) at Mar. 31, 2018					3,132,000
Balance at beginning of period at Dec. 31, 2017	$ 11,201	479,914	4,278,326	2,039	$ (181,130)	4,590,350
Balance (in shares) at Dec. 31, 2017	112,009,000
Balance (in shares) at Dec. 31, 2017					3,164,000
Increase (Decrease) in Shareholders' Equity
Net income (loss)			(11,879)			(11,879)
Other comprehensive income (loss)				(7,260)		(7,260)
Dividends declared			(76,950)			(76,950)
Exercise of employee stock options, net of shares withheld for employee taxes		(1,087)			$ 1,520	433
Exercise of employee stock options, net of shares withheld for employee taxes (in shares)					(29,000)
Vesting of restricted stock awards, net of shares withheld for employee taxes		(151)			$ 151
Vesting of restricted stock awards, net of shares withheld for employee taxes (in shares)					(3,000)
Stock-based compensation		8,459				8,459
Balance at end of period at Mar. 31, 2018	$ 11,201	487,135	4,189,497	(5,221)	$ (179,459)	4,503,153
Balance (in shares) at Mar. 31, 2018	112,009,000
Balance (in shares) at Mar. 31, 2018					3,132,000
Balance at beginning 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
Increase (Decrease) in Shareholders' Equity
Net income (loss)			18,959			$ 18,959
Other comprehensive income (loss)				225		225
Dividends declared			(78,488)			(78,488)
Exercise of employee stock options, net of shares withheld for employee taxes		(6,756)			$ 6,980	224
Exercise of employee stock options, net of shares withheld for employee taxes (in shares)					(125,000)
Vesting of restricted stock awards, net of shares withheld for employee taxes	$ 7	(16,673)			$ 12,129	(4,537)
Vesting of restricted stock awards, net of shares withheld for employee taxes (in shares)	71,000				(215,000)
Stock-based compensation		7,158				7,158
Cumulate effect adjustment for adoption | ASU 2014-09			(38)			(38)
Cumulate effect adjustment for adoption | ASU 2016-01			29,071	(29,071)
Balance at end of period at Dec. 31, 2018	$ 11,208	484,122	3,997,283	(12,296)	$ (154,079)	4,326,238
Balance (in shares) at Dec. 31, 2018	112,080,000
Balance (in shares) at Dec. 31, 2018					2,675,000
Balance at beginning 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
Increase (Decrease) in Shareholders' Equity
Net income (loss)						$ 79,850
Other comprehensive income (loss)						449
Balance at end of period at Mar. 31, 2019	$ 11,208	493,421	3,979,708	(12,072)	$ (153,645)	$ 4,318,620
Balance (in shares) at Mar. 31, 2019	112,080,000					112,080,262
Balance (in shares) at Mar. 31, 2019					2,668,000	2,667,837
Balance at beginning of period at Dec. 31, 2018	$ 11,208	484,122	3,997,283	(12,296)	$ (154,079)	$ 4,326,238
Balance (in shares) at Dec. 31, 2018	112,080,000
Balance (in shares) at Dec. 31, 2018					2,675,000
Increase (Decrease) in Shareholders' Equity
Net income (loss)			60,891			60,891
Other comprehensive income (loss)				224		224
Dividends declared			(78,466)			(78,466)
Exercise of employee stock options, net of shares withheld for employee taxes		(107)			$ 409	302
Exercise of employee stock options, net of shares withheld for employee taxes (in shares)					(7,000)
Vesting of restricted stock awards, net of shares withheld for employee taxes		(25)			$ 25
Stock-based compensation		9,431				9,431
Balance at end of period at Mar. 31, 2019	$ 11,208	$ 493,421	$ 3,979,708	$ (12,072)	$ (153,645)	$ 4,318,620
Balance (in shares) at Mar. 31, 2019	112,080,000					112,080,262
Balance (in shares) at Mar. 31, 2019					2,668,000	2,667,837