CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Operating activities:
Net loss	$ (89,869)	$ (49,882)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation	3,917	3,233
Amortization of right-of-use operating lease assets	394	393
Amortization of patents	544	486
Amortization of premium (discount) on investments, net	4,023	1,062
Amortization of debt issuance costs	860	647
Stock-based compensation expense	37,861	40,790
Gain on investments	(13)	(246)
Non-cash losses related to patents	221	159
Provision for deferred income taxes	0	(2,288)
Changes in operating assets and liabilities:
Contracts receivable	52,807	34,429
Inventories	(234)	(2,181)
Other current and long-term assets	16,481	9,532
Income taxes payable	2	(532)
Accounts payable	(9,569)	411
Accrued compensation	(36,465)	(20,920)
Accrued liabilities and other current liabilities	(11,905)	(3,006)
Deferred contract revenue	(23,717)	(19,679)
Net cash used in operating activities	(54,662)	(7,592)
Investing activities:
Purchases of short-term investments	(330,051)	(544,375)
Proceeds from sale of short-term investments	411,907	459,352
Purchases of property, plant and equipment	(1,772)	(9,080)
Acquisition of licenses and other assets, net	(1,228)	(904)
Net cash provided by (used in) investing activities	78,856	(95,007)
Financing activities:
Proceeds from equity, net	7,760	7,652
Payments of tax withholdings related to vesting of employee stock awards and exercise of employee stock options	(15,337)	(11,603)
Repurchases and retirements of common stock	0	(90,550)
Net cash used in financing activities	(7,577)	(94,501)
Effects of exchange rates on cash	(126)	8
Net increase (decrease) in cash and cash equivalents	16,491	(197,092)
Cash and cash equivalents at beginning of period	397,664	683,287
Cash and cash equivalents at end of period	414,155	486,195
Supplemental disclosures of cash flow information:
Interest paid	594	601
Income taxes paid	2	3
Supplemental disclosures of non-cash investing and financing activities:
Amounts accrued for capital and patent expenditures	$ 1,876	$ 4,903