CONSOLIDATED STATEMENT OF STOCKHOLDERS' EQUITY - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-In Capital	Accumulated Deficit	Accumulated Other Comprehensive Income	Convertible Senior Notes Due 2019	Convertible Senior Notes Due 2019 Additional Paid-In Capital
Balances (in shares) at Dec. 31, 2017		40,669,000
Balance at beginning of period at Dec. 31, 2017	$ 279,483	$ 41	$ 669,032	$ (389,136)	$ (454)
Increase (Decrease) in Stockholders' Equity
Exercise of stock options (in shares)		46,000
Exercise of stock options	419	$ 0	419
Vested restricted stock units (in shares)		5,000
Vested restricted stock units	0	$ 0
Stock-based compensation	8,385		8,385
Net unrealized gain (loss) on investments	(447)				(447)
Net loss	(10,680)			(10,680)
Balances (in shares) at Mar. 31, 2018		40,720,000
Balance at end of period at Mar. 31, 2018	278,521	$ 41	677,836	(398,455)	(901)
Increase (Decrease) in Stockholders' Equity
Cumulative effect adjustment of the adoption of Accounting Standards Update 2016-09 (Note 2)	1,361			1,361
Adjustments to Additional Paid in Capital, Equity Component of Convertible Debt						$ (233)	$ (233)
Balances (in shares) at Dec. 31, 2018		41,223,000
Balance at beginning of period at Dec. 31, 2018	321,226	$ 41	709,691	(388,226)	(280)
Increase (Decrease) in Stockholders' Equity
Exercise of stock options (in shares)		62,000
Exercise of stock options	1,557	$ 0	1,557
Vested restricted stock units (in shares)		4,000
Vested restricted stock units	$ 0	$ 0
Shares issued under employee stock purchase plan (shares)	0
Stock-based compensation	$ 7,434		7,434
Net unrealized gain (loss) on investments	462				462
Net loss	(2,771)			(2,771)
Balances (in shares) at Mar. 31, 2019		41,289,000
Balance at end of period at Mar. 31, 2019	327,519	$ 41	$ 718,449	(391,153)	$ 182
Increase (Decrease) in Stockholders' Equity
Cumulative effect adjustment of the adoption of Accounting Standards Update 2016-09 (Note 2)	$ (156)			$ (156)