Consolidated Statements of Changes in Stockholders' Equity - USD ($) $ in Thousands	Total	Common Stock [Member]	Treasury Stock [Member]	Additional Paid-in Capital [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Accumulated Deficit [Member]
Balances at Dec. 31, 2017	$ 475,475	$ 5	$ (113)	$ 1,066,059	$ (29)	$ (590,447)
Balances, Shares at Dec. 31, 2017		42,002,934	960
Issuance of common stock from exercise of stock options, Amount	11,727			11,727
Issuance of common stock from exercise of stock options, Shares		402,227
Issuance of common stock under employee stock purchase plan	1,127			1,127
Issuance of common stock under employee stock purchase plan, Shares		11,683
Purchase of treasury stock, Amount	(98)		$ (98)
Purchase of treasury stock, Shares			554
Stock-based compensation expense	15,549			15,549
Public offering of common stock, net of offering costs, Amount	631,154			631,154
Public offering of common stock, net of offering costs, Shares		4,032,012
Unrealized loss on available-for-sale securities	(156)				(156)
Vesting of restricted stock units, net of employee tax obligations	(904)			(904)
Vesting of restricted stock units, net of employee tax obligations, Shares		9,442
Net loss	(74,598)					(74,598)
Balances at Mar. 31, 2018	1,059,276	$ 5	$ (211)	1,724,712	(185)	(665,045)
Balances, Shares at Mar. 31, 2018		46,458,298	1,514
Balances at Dec. 31, 2017	475,475	$ 5	$ (113)	1,066,059	(29)	(590,447)
Balances, Shares at Dec. 31, 2017		42,002,934	960
Net loss	(91,576)
Balances at Jun. 30, 2018	1,077,763	$ 5	$ (211)	1,760,137	(145)	(682,023)
Balances, Shares at Jun. 30, 2018		46,603,839	1,514
Balances at Mar. 31, 2018	1,059,276	$ 5	$ (211)	1,724,712	(185)	(665,045)
Balances, Shares at Mar. 31, 2018		46,458,298	1,514
Issuance of common stock from exercise of stock options, Amount	6,643			6,643
Issuance of common stock from exercise of stock options, Shares		145,541
Stock-based compensation expense	28,782			28,782
Unrealized loss on available-for-sale securities	40				40
Net loss	(16,978)					(16,978)
Balances at Jun. 30, 2018	1,077,763	$ 5	$ (211)	1,760,137	(145)	(682,023)
Balances, Shares at Jun. 30, 2018		46,603,839	1,514
Balances at Dec. 31, 2018	862,971	$ 5	$ (211)	1,827,021	(515)	(963,329)
Balances, Shares at Dec. 31, 2018		46,888,263	3,033
Issuance of common stock from exercise of stock options, Amount	14,072			14,072
Issuance of common stock from exercise of stock options, Shares		287,659
Issuance of common stock under employee stock purchase plan	1,799			1,799
Issuance of common stock under employee stock purchase plan, Shares		16,398
Purchase of treasury stock, Amount	(189)		$ (189)
Stock-based compensation expense	43,622			43,622
Public offering of common stock, net of offering costs, Amount	560,948			560,948
Public offering of common stock, net of offering costs, Shares		3,833,334
Unrealized loss on available-for-sale securities	409				409
Vesting of restricted stock units, net of employee tax obligations	(692)			(692)
Vesting of restricted stock units, net of employee tax obligations, Shares		8,518
Net loss	(163,406)					(163,406)
Balances at Mar. 31, 2019	1,319,534	$ 5	$ (400)	2,446,770	(106)	(1,126,735)
Balances, Shares at Mar. 31, 2019		51,034,172	3,033
Balances at Dec. 31, 2018	862,971	$ 5	$ (211)	1,827,021	(515)	(963,329)
Balances, Shares at Dec. 31, 2018		46,888,263	3,033
Net loss	(331,627)
Balances at Jun. 30, 2019	1,202,875	$ 5	$ (400)	2,496,651	1,575	(1,294,956)
Balances, Shares at Jun. 30, 2019		51,469,104	3,033
Balances at Mar. 31, 2019	1,319,534	$ 5	$ (400)	2,446,770	(106)	(1,126,735)
Balances, Shares at Mar. 31, 2019		51,034,172	3,033
Issuance of common stock from exercise of stock options, Amount	15,601			15,601
Issuance of common stock from exercise of stock options, Shares		434,932
Stock-based compensation expense	34,280			34,280
Unrealized loss on available-for-sale securities	1,681				1,681
Net loss	(168,221)					(168,221)
Balances at Jun. 30, 2019	$ 1,202,875	$ 5	$ (400)	$ 2,496,651	$ 1,575	$ (1,294,956)
Balances, Shares at Jun. 30, 2019		51,469,104	3,033