CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
CASH FLOWS FROM OPERATING ACTIVITIES:
Net income	$ 133,372	$ 138,304	$ 58,840
Adjustments to reconcile net income to net cash provided by operating activities:
Amortization of deferred revenue and recognition of unbilled receivables	(183,997)	(135,368)	(68,905)
Depreciation	15,217	12,456	8,612
Amortization of intangibles	21,969	21,962	21,962
Change in excess inventory reserve	1,114	5,938	3,630
Amortization of premium and discount on investments, net	(4,960)	(6,643)	(6,131)
Stock-based compensation to employees	26,631	16,148	12,432
Stock-based compensation to Board of Directors and Scientific Advisory Board	1,647	1,548	4,364
Deferred income tax benefit	(4,446)	(5,776)	(12,814)
Retirement plan expense	5,656	5,818	4,466
Decrease (increase) in assets:
Accounts receivable	(21,809)	(17,323)	9,226
Inventory	(28,752)	109	(37,365)
Other current assets	6,497	(15,238)	4,860
Deferred income taxes			20,682
Other assets	(13,481)	(13,291)	(63,922)
Increase (decrease) in liabilities:
Accounts payable and accrued expenses	(8,305)	15,516	1,563
Other current liabilities	2,683	(5,183)	5,761
Deferred revenue	192,369	157,321	130,639
Other liabilities	7,387	17,614	23,896
Net cash provided by operating activities	148,792	193,912	121,796
CASH FLOWS FROM INVESTING ACTIVITIES:
Purchases of property and equipment	(27,991)	(30,059)	(25,391)
Purchase of intangibles	(60)	(401)
Purchases of investments	(604,153)	(931,854)	(628,789)
Proceeds from sale and maturity of investments	1,023,460	723,600	633,179
Net cash provided by (used in) investing activities	391,256	(238,714)	(21,001)
CASH FLOWS FROM FINANCING ACTIVITIES:
Proceeds from issuance of common stock	1,176	889	798
Repurchase of common stock		(649)	(477)
Payment of withholding taxes related to stock-based compensation to employees	(14,394)	(15,980)	(11,620)
Cash dividends paid	(28,445)	(18,853)	(11,314)
Net cash used in financing activities	(41,663)	(34,593)	(22,613)
INCREASE (DECREASE) IN CASH AND CASH EQUIVALENTS	498,385	(79,395)	78,182
CASH AND CASH EQUIVALENTS, BEGINNING OF YEAR	131,627	211,022	132,840
CASH AND CASH EQUIVALENTS, END OF YEAR	$ 630,012	$ 131,627	$ 211,022