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CONSOLIDATED STATEMENTS OF CASH FLOWS Non-Cash Activities - USD ($)
$ in Thousands
 3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Other Significant Noncash Transactions [Line Items]    
Unrealized gain (loss) on available-for-sale securities $ 69 $ (23)
Net change in accounts payable and accrued expenses related to purchases of property and equipment 4,583 1,928
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
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