Adjustment to additional paid in capital resulting from the recognition of convertible debt instruments as two separate components - a debt component and an equity component. This bifurcation may result in a basis difference associated with the liability component that represents a temporary difference for purposes of applying accounting for income taxes. The initial recognition of deferred taxes for the tax effect of that temporary difference is as an adjustment to additional paid in capital.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 20
-Section 25
-Paragraph 22
-URI http://asc.fasb.org/extlink&oid=51819886&loc=SL6014347-161799

Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	Two Thousand Nineteen Senior Convertible Notes [Member]	Convertible Senior Notes Due 2022 [Member]	Common Stock	Additional Paid-In Capital	Additional Paid-In Capital Two Thousand Nineteen Senior Convertible Notes [Member]	Additional Paid-In Capital Convertible Senior Notes Due 2022 [Member]	Accumulated Deficit	Accumulated Other Comprehensive Income
Balances at Dec. 31, 2014	$ 171,145			$ 36	$ 481,334			$ (310,145)	$ (80)
Balances (in shares) at Dec. 31, 2014				36,151,000
Increase (Decrease) in Stockholders' Equity
Exercise of stock options	10,073			$ 1	10,072
Exercise of stock options (in shares)				618,000
Shares issued under employee stock purchase plan	2,093				2,093
Shares issued under employee stock purchase plan (in shares)				35,000
Stock-based compensation	33,368				33,368
Issuance of common stock upon conversion of 2019 convertible senior notes	3,929				3,929
Issuance of common stock upon conversion of 2019 convertible senior notes (in shares)				44,000
Equity component of convertible senior notes, net of issuance costs	(4,100)				(4,100)
Net unrealized gain (loss) on investments	28								28
Net income (loss)	1,856							1,856
Balances at Dec. 31, 2015	218,392			$ 37	526,696			(308,289)	(52)
Balances (in shares) at Dec. 31, 2015				36,848,000
Increase (Decrease) in Stockholders' Equity
Exercise of stock options	5,770				5,770
Exercise of stock options (in shares)				518,000
Vested restricted stock units	0
Vested restricted stock units (in shares)				62,000
Shares issued under employee stock purchase plan	1,495				1,495
Shares issued under employee stock purchase plan (in shares)				53,000
Stock-based compensation	31,248				31,248
Equity component of convertible senior notes, net of issuance costs	(2)				(2)
Net unrealized gain (loss) on investments	22								22
Net income (loss)	(37,949)							(37,949)
Balances at Dec. 31, 2016	218,976			$ 37	565,207			(346,238)	(30)
Balances (in shares) at Dec. 31, 2016				37,481,000
Increase (Decrease) in Stockholders' Equity
Cumulative effect adjustment of the adoption of Accounting Standards Update 2016-09 (Note 3)					287			(287)
Exercise of stock options	6,778			$ 1	6,777
Exercise of stock options (in shares)				540,000
Vested restricted stock units	0
Vested restricted stock units (in shares)				101,000
Shares issued under employee stock purchase plan	$ 1,862				1,862
Shares issued under employee stock purchase plan (in shares)	160,147			57,000
Stock-based compensation	$ 31,601				31,601
Issuance of common stock upon conversion of 2019 convertible senior notes	120,960			$ 3	120,957
Issuance of common stock upon conversion of 2019 convertible senior notes (in shares)				2,490,000
Equity component of convertible senior notes, net of issuance costs		$ (126,328)	$ 68,669			$ (126,328)	$ 68,669
Net unrealized gain (loss) on investments	(424)								(424)
Net income (loss)	(42,611)							(42,611)
Balances at Dec. 31, 2017	$ 279,483			$ 41	$ 669,032			$ (389,136)	$ (454)
Balances (in shares) at Dec. 31, 2017				40,669,000