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CONSOLIDATED STATEMENTS OF CASH FLOWS Non-Cash Activities - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Other Significant Noncash Transactions [Line Items]      
Unrealized gain (loss) on available-for-sale securities $ 241 $ 342 $ (19)
Net change in accounts payable and accrued expenses related to purchases of property and equipment (530) 3,490 4,363
Cash paid for income tax 46,602 17,771 23,248
Common Stock Issued to Board of Directors and Scientific Advisory Board that was Earned and Accrued for in Previous Period [Member]      
Other Significant Noncash Transactions [Line Items]      
Other significant noncash transaction, value of consideration received $ 300 $ 300 300
Common Stock Issued to Employees that was Earned and Accrued for in Previous Period Net of Shares Withheld for Taxes [Member]      
Other Significant Noncash Transactions [Line Items]      
Other significant noncash transaction, value of consideration received     $ 174