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Summary of Significant Accounting Policies (Details) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018 USD ($) RevenueStream $ / shares	Jun. 30, 2017 USD ($) $ / shares	Jun. 30, 2018 USD ($) RevenueStream $ / shares	Jun. 30, 2017 USD ($) $ / shares	Dec. 31, 2017 USD ($)
Condensed Consolidated Balance Sheets [Abstract]
Current portion of deferred revenue	$ 35,713		$ 35,713		$ 58,192
Long-term portion of deferred revenue	5,839		5,839		12,501
Accumulated deficit	(387,894)		(387,894)		(296,221)
Condensed Consolidated Statements of Operations and Comprehensive Loss [Abstract]
Revenue	18,321	$ 5,713	35,429	$ 11,807
Loss from operations	(63,423)	(19,689)	(93,750)	(83,065)
Net loss	(62,046)	(20,359)	(91,673)	(84,214)
Condensed Consolidated Statement of Cash Flows [Abstract]
Net loss	(62,046)	(20,359)	(91,673)	(84,214)
Adjustments to reconcile net loss to net cash provided by operating activities:
Deferred revenue			(29,141)	63,193
Cash, cash equivalents at beginning of period			58,367	7,857
Cash and cash equivalents	216,840	75,445	216,840	75,445
Preferred Stock [Member]
Condensed Consolidated Statements of Operations and Comprehensive Loss [Abstract]
Net loss	$ 0	$ (20,359)	$ 0	$ (84,214)
Net loss per share, basic and diluted (in dollars per share) \| $ / shares	$ 0	$ (0.70)	$ 0	$ (2.92)
Condensed Consolidated Statement of Cash Flows [Abstract]
Net loss	$ 0	$ (20,359)	$ 0	$ (84,214)
Research and Development Under Collaborative Agreement [Member]
Condensed Consolidated Statements of Operations and Comprehensive Loss [Abstract]
Revenue	$ 18,321	5,713	$ 35,429	11,807
Novartis [Member]
Summary of Significant Accounting Policies [Abstract]
Number of revenue streams \| RevenueStream	1		1
Novartis [Member] \| Research and Development Under Collaborative Agreement [Member]
Condensed Consolidated Statements of Operations and Comprehensive Loss [Abstract]
Revenue	$ 18,300	5,700	$ 35,400	11,800
As Originally Reported Under Topic 605 [Member] \| ASU 2014-09 [Member]
Condensed Consolidated Balance Sheets [Abstract]
Current portion of deferred revenue					50,579
Long-term portion of deferred revenue					8,306
Accumulated deficit					(284,413)
Condensed Consolidated Statements of Operations and Comprehensive Loss [Abstract]
Loss from operations		(11,274)		(71,147)
Net loss		(11,944)		(72,296)
Condensed Consolidated Statement of Cash Flows [Abstract]
Net loss		(11,944)		(72,296)
Adjustments to reconcile net loss to net cash provided by operating activities:
Deferred revenue				51,275
Cash, cash equivalents at beginning of period				7,857
Cash and cash equivalents		$ 75,445		$ 75,445
As Originally Reported Under Topic 605 [Member] \| ASU 2014-09 [Member] \| Preferred Stock [Member]
Condensed Consolidated Statements of Operations and Comprehensive Loss [Abstract]
Net loss per share, basic and diluted (in dollars per share) \| $ / shares		$ (0.41)		$ (2.50)
As Originally Reported Under Topic 605 [Member] \| ASU 2014-09 [Member] \| Research and Development Under Collaborative Agreement [Member]
Condensed Consolidated Statements of Operations and Comprehensive Loss [Abstract]
Revenue		$ 14,128		$ 23,725
Effect of Change [Member] \| ASU 2014-09 [Member]
Condensed Consolidated Balance Sheets [Abstract]
Current portion of deferred revenue					7,613
Long-term portion of deferred revenue					4,195
Accumulated deficit					$ (11,808)
Condensed Consolidated Statements of Operations and Comprehensive Loss [Abstract]
Loss from operations		(8,415)		(11,918)
Net loss		(8,415)		(11,918)
Condensed Consolidated Statement of Cash Flows [Abstract]
Net loss		(8,415)		(11,918)
Adjustments to reconcile net loss to net cash provided by operating activities:
Deferred revenue				11,918
Cash, cash equivalents at beginning of period				0
Cash and cash equivalents		$ 0		$ 0
Effect of Change [Member] \| ASU 2014-09 [Member] \| Preferred Stock [Member]
Condensed Consolidated Statements of Operations and Comprehensive Loss [Abstract]
Net loss per share, basic and diluted (in dollars per share) \| $ / shares		$ (0.29)		$ (0.42)
Effect of Change [Member] \| ASU 2014-09 [Member] \| Research and Development Under Collaborative Agreement [Member]
Condensed Consolidated Statements of Operations and Comprehensive Loss [Abstract]
Revenue		$ (8,415)		$ (11,918)