CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-in Capital	Accumulated Other Comprehensive Loss	Accumulated Deficit
Beginning balance (in shares) at Dec. 31, 2017		34,083,112
Beginning balance at Dec. 31, 2017	$ 790,244	$ 341	$ 1,271,628	$ (3,908)	$ (477,817)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net shares issued in connection with the exercise of stock options and vesting of restricted stock units, net of withholdings (in shares)		306,956
Net shares issued in connection with the exercise of stock options and vesting of restricted stock units, net of withholdings	(889)	$ 3	(892)
Non-cash equity based compensation	11,122		11,122
Unrealized losses on securities, net of tax	(387)			(387)
Net loss	(74,324)				(74,324)
Ending balance (in shares) at Jun. 30, 2018		34,390,068
Ending balance at Jun. 30, 2018	726,904	$ 344	1,281,858	(4,295)	(551,003)
Beginning balance (in shares) at Mar. 31, 2018		34,322,193
Beginning balance at Mar. 31, 2018	739,994	$ 343	1,274,935	(4,362)	(530,922)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net shares issued in connection with the exercise of stock options and vesting of restricted stock units, net of withholdings (in shares)		67,875
Net shares issued in connection with the exercise of stock options and vesting of restricted stock units, net of withholdings	1,335	$ 1	1,334
Non-cash equity based compensation	5,589		5,589
Unrealized losses on securities, net of tax	67			67
Net loss	(20,081)				(20,081)
Ending balance (in shares) at Jun. 30, 2018		34,390,068
Ending balance at Jun. 30, 2018	$ 726,904	$ 344	1,281,858	(4,295)	(551,003)
Beginning balance (in shares) at Dec. 31, 2018	34,606,760	34,606,760
Beginning balance at Dec. 31, 2018	$ 746,655	$ 346	1,292,736	(3,985)	(542,442)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net shares issued in connection with the exercise of stock options and vesting of restricted stock units, net of withholdings (in shares)		262,919
Net shares issued in connection with the exercise of stock options and vesting of restricted stock units, net of withholdings	(1,718)	$ 3	(1,721)
Issuance of common stock under employee stock purchase plan (in shares)		105,075
Issuance of common stock under employee stock purchase plan	851	$ 1	850
Repurchase of common stock pursuant to the 2016 share repurchase program (in shares)		(1,074,800)
Repurchase of common stock pursuant to the share repurchase program	(13,730)	$ (11)	(13,719)
Non-cash equity based compensation	9,407		9,407
Unrealized losses on securities, net of tax	953			953
Net loss	$ (242,911)				(242,911)
Ending balance (in shares) at Jun. 30, 2019	33,899,954	33,899,954
Ending balance at Jun. 30, 2019	$ 499,507	$ 339	1,287,553	(3,032)	(785,353)
Beginning balance (in shares) at Mar. 31, 2019		33,746,828
Beginning balance at Mar. 31, 2019	614,719	$ 337	1,282,284	(3,376)	(664,526)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net shares issued in connection with the exercise of stock options and vesting of restricted stock units, net of withholdings (in shares)		48,051
Net shares issued in connection with the exercise of stock options and vesting of restricted stock units, net of withholdings	(114)	$ 1	(115)
Issuance of common stock under employee stock purchase plan (in shares)		105,075
Issuance of common stock under employee stock purchase plan	851	$ 1	850
Non-cash equity based compensation	4,534		4,534
Unrealized losses on securities, net of tax	344			344
Net loss	$ (120,827)				(120,827)
Ending balance (in shares) at Jun. 30, 2019	33,899,954	33,899,954
Ending balance at Jun. 30, 2019	$ 499,507	$ 339	$ 1,287,553	$ (3,032)	$ (785,353)