Condensed Consolidated Statements of Comprehensive Income (Loss) (Unaudited) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Statement Of Income And Comprehensive Income [Abstract]
Net income (loss)	$ 4,004	$ (33,287)	$ (4,984)	$ (42,127)
Other comprehensive income (loss):
Unrealized gain (loss) on available-for-sale securities	5	(3)	5	(8)
Comprehensive income (loss)	$ 4,009	$ (33,290)	$ (4,979)	$ (42,135)