CONSOLIDATED STATEMENTS OF CASH FLOWS (UNAUDITED) - USD ($)	6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019
CASH FLOWS FROM OPERATING ACTIVITIES:
Net income	$ 38,970,000	$ 74,914,000
Adjustments to reconcile net income to net cash provided by operating activities:
Amortization of deferred revenue and recognition of unbilled receivables	(58,644,000)	(63,942,000)
Depreciation	7,283,000	5,799,000
Amortization of intangibles	10,980,000	10,976,000
Change in excess inventory reserve	611,000	4,155,000
Amortization of premium and discount on investments, net	(3,280,000)	(3,150,000)
Stock-based compensation to employees	12,918,000	7,106,000
Stock-based compensation to Board of Directors and Scientific Advisory Board	708,000	884,000
Deferred income tax expense (benefit)	(336,000)	(1,218,000)
Retirement plan expense	2,828,000	2,940,000
Decrease (increase) in assets:
Accounts receivable	(8,046,000)	(23,823,000)
Inventory	(21,181,000)	5,707,000
Other current assets	(5,967,000)	(2,816,000)
Other assets	(5,942,000)	(5,846,000)
Increase (decrease) in liabilities:
Accounts payable and accrued expenses	(20,332,000)	(2,099,000)
Other current liabilities	(639,000)	6,736,000
Deferred revenue	75,683,000	68,213,000
Other liabilities	6,416,000	(657,000)
Net cash provided by operating activities	32,030,000	83,879,000
CASH FLOWS FROM INVESTING ACTIVITIES:
Purchases of property and equipment	(11,804,000)	(20,322,000)
Purchases of intangibles	(25,000)	(401,000)
Purchases of investments	(404,232,000)	(363,595,000)
Proceeds from sale of investments	412,760,000	437,800,000
Net cash (used in) provided by investing activities	(3,301,000)	53,482,000
CASH FLOWS FROM FINANCING ACTIVITIES:
Proceeds from issuance of common stock	590,000	445,000
Repurchase of common stock		(649,000)
Payment of withholding taxes related to stock-based compensation to employees	(13,165,000)	(14,472,000)
Cash dividends paid	(14,215,000)	(9,430,000)
Net cash used in financing activities	(26,790,000)	(24,106,000)
INCREASE IN CASH AND CASH EQUIVALENTS	1,939,000	113,255,000
CASH AND CASH EQUIVALENTS, BEGINNING OF PERIOD	131,627,000	211,022,000
CASH AND CASH EQUIVALENTS, END OF PERIOD	$ 133,566,000	$ 324,277,000