Consolidated Statements of Shareholders' Equity Statement - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock	AOCI Attributable to Parent	Retained Earnings
Beginning Balance (in shares) at Sep. 30, 2018		60,215
Beginning Balance at Sep. 30, 2018	$ 1,285,492	$ 20,427	$ (22,178)	$ 1,287,243
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Exercise of employee stock options (in shares)		1
Exercise of employee stock options	16	$ 16
Issuance of stock under employee stock purchase plan (in shares)		334
Issuance of stock under employee stock purchase plan	45,439	$ 45,439
Issuance of restricted stock (in shares)		765
Issuance of restricted stock	0
Repurchase of common stock (in shares)		(1,186)
Repurchase of common stock	(201,045)	$ (88,110)		(112,935)
Stock-based compensation	120,950	$ 120,950
Net income	332,897			332,897
Other comprehensive income	3,985		3,985
Ending Balance ( in shares) at Jun. 30, 2019		60,129
Ending Balance at Jun. 30, 2019	1,623,782	$ 98,722	(18,193)	1,543,253
Beginning Balance (in shares) at Mar. 31, 2019		59,695
Beginning Balance at Mar. 31, 2019	1,467,408	$ 29,401	(19,341)	1,457,348
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Exercise of employee stock options (in shares)		1
Exercise of employee stock options	16	$ 16
Issuance of stock under employee stock purchase plan (in shares)		199
Issuance of stock under employee stock purchase plan	26,539	$ 26,539
Issuance of restricted stock (in shares)		234
Issuance of restricted stock	0
Stock-based compensation	42,766	$ 42,766
Net income	85,905			85,905
Other comprehensive income	1,148		1,148
Ending Balance ( in shares) at Jun. 30, 2019		60,129
Ending Balance at Jun. 30, 2019	$ 1,623,782	$ 98,722	(18,193)	1,543,253
Beginning Balance (in shares) at Sep. 30, 2019	60,367	60,367
Beginning Balance at Sep. 30, 2019	$ 1,761,497	$ 142,597	(19,190)	1,638,090
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Exercise of employee stock options (in shares)		72
Exercise of employee stock options	1,760	$ 1,760
Issuance of stock under employee stock purchase plan (in shares)		419
Issuance of stock under employee stock purchase plan	50,239	$ 50,239
Issuance of restricted stock (in shares)		748
Issuance of restricted stock	0
Repurchase of common stock (in shares)		(442)
Repurchase of common stock	(50,009)	$ (50,009)
Stock-based compensation	159,939	$ 159,939
Net income	229,778			229,778
Other comprehensive income	$ 695		695
Ending Balance ( in shares) at Jun. 30, 2020	61,164	61,164
Ending Balance at Jun. 30, 2020	$ 2,153,899	$ 304,526	(18,495)	1,867,868
Beginning Balance (in shares) at Mar. 31, 2020		60,629
Beginning Balance at Mar. 31, 2020	1,998,194	$ 223,101	(22,903)	1,797,996
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Exercise of employee stock options (in shares)		41
Exercise of employee stock options	987	$ 987
Issuance of stock under employee stock purchase plan (in shares)		250
Issuance of stock under employee stock purchase plan	29,570	$ 29,570
Issuance of restricted stock (in shares)		244
Issuance of restricted stock	0
Stock-based compensation	50,868	$ 50,868
Net income	69,872			69,872
Other comprehensive income	$ 4,408		4,408
Ending Balance ( in shares) at Jun. 30, 2020	61,164	61,164
Ending Balance at Jun. 30, 2020	$ 2,153,899	$ 304,526	$ (18,495)	$ 1,867,868