Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Cash flows from operating activities
Net income	$ 46,383	$ 71,956	$ 17,710
Adjustments to reconcile net income to net cash provided by operating activities:
Stock-based compensation expense	19,226	15,845	13,071
Depreciation and amortization expense	3,258	2,518	2,137
Deferred income taxes	(3,138)	1,858	(1,654)
Income tax benefit from stock awards			(213)
Premium paid on marketable securities	(1,491)	(319)	(1,229)
(Accretion) amortization of (discount) premium on marketable securities	(4,336)	(835)	702
Change in fair value of warrant liability and Series 1 nonconvertible preferred stock		59	159
Other non-cash items	25	(75)
Change in operating assets and liabilities:
Accounts receivable	15,892	(56,591)	2,227
Prepaid expenses and other current assets	(10,845)	(918)	5,678
Accounts payable	1,791	1,317	633
Accrued expenses	5,750	1,843	3,443
Income taxes payable	(1,388)	(7,910)	9,511
Other long-term liabilities	291	564	478
Other long-term assets		(92)
Net cash provided by operating activities	71,418	29,220	52,653
Cash flows from investing activities
Purchase of marketable securities	(549,312)	(293,103)	(251,371)
Proceeds from maturities and sale of marketable securities	468,065	260,682	249,305
Purchase of property and equipment	(5,417)	(2,981)	(2,506)
Net cash used in investing activities	(86,664)	(35,402)	(4,572)
Cash flows from financing activities
Proceeds from exercise of stock options and warrants	6,848	6,244	1,079
Income tax benefit from exercise of stock options			213
Payments for settlement of share-based awards	(4,188)	(1,756)	(202)
Payments of capital lease obligations	(86)	(79)	(73)
Net cash provided by financing activities	2,574	4,409	1,017
Net (decrease) increase in cash and cash equivalents	(12,672)	(1,773)	49,098
Cash, cash equivalents and restricted cash at beginning of period	64,510	66,283	17,185
Cash, cash equivalents and restricted cash at end of period	51,838	64,510	66,283
Supplemental disclosure of cash flow information:
Cash paid for income taxes	12,672	26,088	1,588
Non-cash items:
Purchases of fixed assets included in accounts payable and accrued expenses	$ 320	$ 111	$ 318