Prior-Period Financial Statements - Cash Flows (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
Cash flows from operating activities
Net loss	$ (55,361)	$ (23,200)
Adjustments to reconcile net loss to net cash used in operating activities
Depreciation and amortization	1,552	864
Amortization of premium on marketable securities	406	1,356
Stock-based compensation	9,353	6,607
Abandonment of capitalized intangible assets	50	225
Changes in operating assets and liabilities:
Accounts receivable	(140)	(6,260)
Interest receivable	(595)	(158)
Prepaid expenses and other assets	(6,468)	(4,727)
Accounts payable	(1,953)	3,500
Accrued expenses	443	(2,708)
Income taxes	(157)	390
Deferred rent	206	(12)
Deferred revenue		701
Net cash used in operating activities	(52,664)	(23,422)
Cash flows from investing activities
Purchase of marketable securities	(285,809)	(33,440)
Purchase of intangible assets	(882)	(1,092)
Purchase of property and equipment	(3,416)	(1,256)
Proceeds from sale and maturities of marketable securities	112,526	53,866
Repayments of loan	86	(86)
Net cash provided by (used in) investing activities	(177,495)	17,992
Cash flows from financing activities
Proceeds from issuance of common stock upon exercise of stock awards	2,915	1,680
Proceeds from issuance of common stock under the Employee Stock Purchase Plan	504	442
Proceeds from issuance of common stock	260,245
Common stock issuance cost	(14,741)
Net cash provided by financing activities	248,923	2,122
Net increase (decrease) in cash and cash equivalents	18,764	(3,308)
Cash and cash equivalents, beginning of period	16,528	14,528
Cash and cash equivalents, end of period	$ 35,292	11,220
ASU 2014-09 | As Reported Before ASC Topic 606
Cash flows from operating activities
Net loss		(21,519)
Adjustments to reconcile net loss to net cash used in operating activities
Depreciation and amortization		864
Amortization of premium on marketable securities		1,356
Stock-based compensation		6,607
Abandonment of capitalized intangible assets		225
Changes in operating assets and liabilities:
Accounts receivable		(6,260)
Interest receivable		(144)
Prepaid expenses and other assets		(4,727)
Accounts payable		3,500
Accrued expenses		(2,708)
Income taxes		390
Deferred rent		(12)
Deferred revenue		(1,080)
Net cash used in operating activities		(23,508)
Cash flows from investing activities
Purchase of marketable securities		(33,440)
Purchase of intangible assets		(1,092)
Purchase of property and equipment		(1,256)
Proceeds from sale and maturities of marketable securities		53,866
Net cash provided by (used in) investing activities		18,078
Cash flows from financing activities
Proceeds from issuance of common stock upon exercise of stock awards		1,680
Proceeds from issuance of common stock under the Employee Stock Purchase Plan		442
Net cash provided by financing activities		2,122
Net increase (decrease) in cash and cash equivalents		(3,308)
Cash and cash equivalents, beginning of period		14,528
Cash and cash equivalents, end of period		11,220
ASU 2014-09 | Effect of Adoption of ASC Topic 606
Cash flows from operating activities
Net loss		(1,681)
Changes in operating assets and liabilities:
Interest receivable		(14)
Deferred revenue		1,781
Net cash used in operating activities		86
Cash flows from investing activities
Repayments of loan		(86)
Net cash provided by (used in) investing activities		$ (86)