Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	Common Shares [Member]	Additional Paid-In Capital [Member]	Accumulated Deficit [Member]
Beginning Balance at Dec. 31, 2013	$ 87,808	$ 26	$ 207,503	$ (119,721)
Beginning Balance, shares at Dec. 31, 2013		25,629,000
Exercise of stock options	1,421		1,421
Exercise of stock options, shares		563,000
Issuance of common stock from initial public offering (net of underwriters discounts and issuance costs)	80,766	$ 5	80,761
Issuance of common stock from initial public offering, shares		4,543,000
Reclassification of warrants for common stock	83		83
Reclassification of warrants for common stock, shares		8,000
Issuance of common stock from warrant exercise, shares		63,000
Stock-based compensation expense	5,230		5,230
Net loss	(66,742)			(66,742)
Ending Balance at Dec. 31, 2014	108,566	$ 31	294,998	(186,463)
Ending Balance, shares at Dec. 31, 2014		30,806,000
Exercise of stock options	4,672	$ 1	4,671
Exercise of stock options, shares		818,000
Issuance of common stock from follow-on public offering (net of underwriters discounts and issuance costs)	162,151	$ 5	162,146
Issuance of common stock from follow-on public offering, shares		4,945,000
Shares issued in connection with employee stock purchase plan	239		239
Shares issued in connection with employee stock purchase plan, shares		16,000
Stock-based compensation expense	11,616		11,616
Net loss	(83,189)			(83,189)
Ending Balance at Dec. 31, 2015	$ 204,055	$ 37	473,670	(269,652)
Ending Balance, shares at Dec. 31, 2015		36,585,000
Exercise of stock options, shares	93,447
Exercise of stock options and vesting of restricted stock units	$ 146		146
Exercise of stock options and vesting of restricted stock units, shares		298,000
Shares issued in connection with employee stock purchase plan	187		187
Shares issued in connection with employee stock purchase plan, shares		59,000
Stock-based compensation expense	13,145		13,145
Net loss	(77,480)			(77,480)
Ending Balance at Dec. 31, 2016	$ 140,053	$ 37	$ 487,148	$ (347,132)
Ending Balance, shares at Dec. 31, 2016		36,942,000