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	Common stock	Additional paid-in capital	Accumulated other comprehensive (loss) income	Accumulated deficit
Balance at the beginning of the period at Dec. 31, 2014	$ 298,467	$ 33	$ 721,722	$ (737)	$ (422,551)
Balance (in shares) at Dec. 31, 2014		32,898,392
Increase (Decrease) in Stockholders' Equity
Equity component of the convertible notes issuance, net	55,754		55,754
Exercise of options	8,711	$ 1	8,710
Exercise of options (in shares)		656,248
Restricted stock vesting and issuance	0
Restricted stock vesting and issuance (in shares)		361,919
Share-based compensation expense	33,979		33,979
Net loss	(170,447)				(170,447)
Comprehensive loss	(463)			(463)
Balance at the end of the period at Dec. 31, 2015	226,001	$ 34	820,165	(1,200)	(592,998)
Balance (in shares) at Dec. 31, 2015		33,916,559
Increase (Decrease) in Stockholders' Equity
Exercise of options	968	$ 0	968
Exercise of options (in shares)		89,216
Restricted stock vesting and issuance	0
Restricted stock vesting and issuance (in shares)		163,635
Share-based compensation expense	35,009		35,009
Net loss	(142,110)				(142,110)
Comprehensive loss	(285)			(285)
Balance at the end of the period at Dec. 31, 2016	119,583	$ 34	856,142	(1,485)	(735,108)
Balance (in shares) at Dec. 31, 2016		34,169,410
Increase (Decrease) in Stockholders' Equity
Issuance of common stock related to acquisition	75,191	$ 7	75,184
Issuance of common stock related to acquisition (in shares)		6,683,598
Exercise of options	2,182	$ 0	2,182
Exercise of options (in shares)		202,085
Restricted stock vesting and issuance	1	$ 1
Restricted stock vesting and issuance (in shares)		287,531
Issuance of common stock in connection with an employee stock purchase plan	2,467	$ 0	2,467
Issuance of common stock in connection with an employee stock purchase plan (in shares)		269,771
Share-based compensation expense	30,559		30,559
Net loss	(79,000)				(79,000)
Comprehensive loss	5,454			5,454
Balance at the end of the period at Dec. 31, 2017	$ 156,437	$ 42	$ 966,534	$ 3,969	$ (814,108)
Balance (in shares) at Dec. 31, 2017		41,612,395