CONSOLIDATED STATEMENTS OF CASH FLOWS Non-Cash Activities - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
Other Significant Noncash Transactions [Line Items]
Unrealized gain on available-for-sale securities	$ 219	$ 140
Net change in accounts payable and accrued expenses related to purchases of property and equipment	3,100	4,169
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