STATEMENTS OF CASH FLOWS (Parenthetical) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statement Of Cash Flows [Abstract]
Interest received	$ 7	$ 31	$ 18
Interest paid	137	44	15
Income taxes paid	$ 108	$ 0	$ 0