CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Statement of Cash Flows [Abstract]
Proceeds from Sale and Maturity of Other Investments	$ 20,000	$ 0	$ 0
Cash flows from operating activities:
Net income	142,415	128,256	109,448
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	30,182	36,214	31,633
Share-based compensation	74,814	61,222	55,509
Deferred income taxes	25,729	(6,049)	(26,007)
Tax effect from share-based payment arrangements	0	0	24,184
Provision of doubtful accounts	623	1,640	2,011
Net gain on marketable securities	(1,449)	0	0
Gain on sale of cost-method investment	(10,000)	0	0
Net loss on sales of property and equipment	231	14	6
Changes in operating assets and liabilities:
Accounts receivable	(42,403)	(1,265)	(18,225)
Prepaid expenses and other assets	(8,504)	(7,115)	12,848
Accounts payable	843	(2,027)	564
Accrued compensation and employee benefits	7,352	6,464	17,079
Other liabilities	6,416	(683)	(4,282)
Deferred revenue	(3,197)	8,973	5,500
Net cash provided by operating activities	223,052	225,644	210,268
Cash flows from investing activities:
Purchases of property and equipment	(31,299)	(19,828)	(21,969)
Proceeds from sales of marketable securities	3,230	0	0
Purchases of marketable securities	(6,050)	0	0
Cash paid for acquisitions, net of cash acquired	0	0	(5,683)
Distribution from (purchase of) cost-method investment	0	(777)	37
Net cash used in investing activities	(14,119)	(20,605)	(27,615)
Cash flows from financing activities:
Proceeds from revolving line of credit	427,000	190,000	122,000
Payments on revolving line of credit	(531,000)	(84,000)	(99,000)
Proceeds from issuance of senior notes	400,000	0	0
Payments on senior notes	(131,000)	(72,000)	(60,000)
Payments on debt issuance costs	(7,849)	0	0
Proceeds from issuance of treasury stock under employee stock plans	11,023	14,474	17,828
Taxes paid related to net share settlement of equity awards	(44,205)	(40,232)	(29,955)
Dividends paid	0	(1,238)	(2,489)
Repurchases of common stock	(342,596)	(187,629)	(138,399)
Net cash used in financing activities	(218,627)	(180,625)	(190,015)
Effect of exchange rate changes on cash	(5,901)	5,278	(2,832)
Increase (decrease) in cash and cash equivalents	(15,595)	29,692	(10,194)
Cash and cash equivalents, beginning of year	105,618	75,926	86,120
Cash and cash equivalents, end of year	90,023	105,618	75,926
Supplemental disclosures of cash flow information:
Cash paid for income taxes, net of refunds of $3,079, $3,757 and $11,363 during the years ended September 30, 2018, 2017 and 2016, respectively	13,398	31,315	10,855
Cash paid for interest	26,106	26,083	26,884
Supplemental disclosures of non-cash investing and financing activities:
Unsettled repurchases of common stock	0	5,661	0
Purchase of property and equipment included in accounts payable	$ 1,913	$ 1,751	$ 3,287