Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-In Capital Amount	Accumulated Other Comprehensive Income (Loss)	Accumulated Deficit
Balance (in shares) at Dec. 31, 2015		42,984,243
Balance at Dec. 31, 2015	$ 461,285	$ 4	$ 907,040	$ 5	$ (445,764)
Increase (Decrease) in Stockholders' Equity
Net loss	(182,804)				(182,804)
Unrealized gain (loss) from available-for-sale securities	66			66
Exercise of warrants (in shares)		19,172
Exercise of options (in shares)		137,719
Exercise of options	2,573		2,573
Share-based compensation expense	26,058		26,058
Balance (in shares) at Dec. 31, 2016		43,141,134
Balance at Dec. 31, 2016	307,178	$ 4	935,671	71	(628,568)
Increase (Decrease) in Stockholders' Equity
Net loss	(254,237)				(254,237)
Unrealized gain (loss) from available-for-sale securities	(385)			(385)
Exercise of options (in shares)		1,385,120
Exercise of options	17,477		17,477
Share-based compensation expense	33,839		33,839
Vesting of restricted shares (in shares)		14,052
Share-based compensation expense related to share-based awards for employee stock purchase plan	1,164		1,164
Issuance of common stock upon purchase by employee stock purchase plan (in shares)		76,280
Issuance of common stock upon purchase by employee stock purchase plan	2,550		2,550
Equity component of 2024 Notes	138,707		138,707
Equity component of deferred financing costs for 2024 Notes	(4,257)		(4,257)
Balance (in shares) at Dec. 31, 2017		44,616,586.000
Balance at Dec. 31, 2017	242,036	$ 4	1,124,630	(314)	(882,284)
Increase (Decrease) in Stockholders' Equity
Net loss	(221,337)				(221,337)
Unrealized gain (loss) from available-for-sale securities	(441)			(441)
Exercise of warrants (in shares)		336,059
Exercise of options (in shares)		472,374
Exercise of options	9,107	$ 1	9,106
Share-based compensation expense	27,905		27,905
Vesting of restricted shares (in shares)		36,852
Vesting of restricted shares	0
Share-based compensation expense related to share-based awards for employee stock purchase plan	796		796
Issuance of common stock upon purchase by employee stock purchase plan (in shares)		101,822
Issuance of common stock upon purchase by employee stock purchase plan	2,566		2,566
Balance (in shares) at Dec. 31, 2018		45,563,693.000
Balance at Dec. 31, 2018	$ 60,632	$ 5	$ 1,165,003	$ (755)	$ (1,103,621)