Summary of Significant Accounting Policies - Impact of Adopting ASC 606, Adjusted Balance Sheet (Details) - USD ($) $ in Thousands		1 Months Ended	3 Months Ended		6 Months Ended	12 Months Ended
	Jan. 02, 2018	May 31, 2018	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2018	Dec. 31, 2017	Dec. 31, 2010	Jun. 30, 2017	Dec. 31, 2016	Dec. 31, 2015
ASSETS
Cash and cash equivalents			$ 345,058		$ 345,058	$ 267,107		$ 150,337	$ 159,964
Accounts receivable			19		19	2,649
Unbilled revenue			522		522	2,580
Non-cash royalty receivable			7,236		7,236
Inventory			1,890		1,890	1,038
Prepaid and other current assets			9,893		9,893	2,967
Total current assets			364,618		364,618	276,341
Property and equipment, net of accumulated depreciation			12,029		12,029	14,538
Other assets			4,437		4,437	3,797
Total assets			381,084		381,084	294,676
LIABILITIES AND SHAREHOLDERS’ EQUITY (DEFICIT)
Accounts payable			12,881		12,881	8,562
Accrued compensation			8,524		8,524	11,473
Other accrued liabilities			17,865		17,865	15,767
Current portion of deferred lease incentive			793		793	784
Current portion of liability related to the sale of future royalties, net			22,265		22,265	17,779
Current portion of deferred revenue			1,020		1,020	1,405
Total current liabilities			63,348		63,348	55,770
Deferred lease incentive, net of current portion			4,806		4,806	5,129				$ 2,000
Deferred revenue, net of current portion			80,751		80,751	93,752
Convertible 4.5% senior notes, net of deferred financing costs of $43 and $50, respectively			2,057		2,057	2,050
Liability related to the sale of future royalties, net			136,701		136,701	151,634
Other long-term liabilities			4,231		4,231	4,236
Total liabilities			291,894		291,894	312,571
Shareholders' deficit:
Preferred stock
Common stock			1,490		1,490	1,325
Additional paid-in capital			1,182,429		1,182,429	1,009,362
Accumulated deficit			(1,094,729)		(1,094,729)	(1,028,582)
Total shareholders’ equity (deficit)			89,190		89,190	(17,895)
Total liabilities and shareholders’ equity (deficit)			381,084		381,084	294,676
Reduction to accumulated deficit related to previously delivered license	$ 4,600
Deferred tax assets:
Milestone recognized as contract asset					4,041
Novartis
Deferred tax assets:
Potential milestone payment							$ 199,500
Takeda
LIABILITIES AND SHAREHOLDERS’ EQUITY (DEFICIT)
Deferred revenue, net of current portion			10,900		10,900
Pro forma as if previous accounting was in effect | ASU 2014-09
ASSETS
Cash and cash equivalents	267,107		345,058		345,058
Accounts receivable	2,649		19		19
Unbilled revenue	2,580		522		522
Non-cash royalty receivable	8,900
Inventory	1,038		1,890		1,890
Prepaid and other current assets	2,967		9,893		9,893
Total current assets	285,241		357,382		357,382
Property and equipment, net of accumulated depreciation	14,538		12,029		12,029
Other assets	3,797		4,437		4,437
Total assets	303,576		373,848		373,848
LIABILITIES AND SHAREHOLDERS’ EQUITY (DEFICIT)
Accounts payable	8,562		12,881		12,881
Accrued compensation	11,473		8,524		8,524
Other accrued liabilities	15,767		17,865		17,865
Current portion of deferred lease incentive	784		793		793
Current portion of liability related to the sale of future royalties, net	17,779		22,265		22,265
Current portion of deferred revenue	1,446		755		755
Total current liabilities	55,811		63,083		63,083
Deferred lease incentive, net of current portion	5,129		4,806		4,806
Deferred revenue, net of current portion	88,521		83,908		83,908
Convertible 4.5% senior notes, net of deferred financing costs of $43 and $50, respectively	2,050		2,057		2,057
Liability related to the sale of future royalties, net	151,634		136,701		136,701
Other long-term liabilities	4,236		4,231		4,231
Total liabilities	307,381		294,786		294,786
Shareholders' deficit:
Common stock	1,325		1,490		1,490
Additional paid-in capital	1,009,362		1,182,429		1,182,429
Accumulated deficit	(1,014,492)		(1,104,857)		(1,104,857)
Total shareholders’ equity (deficit)	(3,805)		79,062		79,062
Total liabilities and shareholders’ equity (deficit)	$ 303,576		373,848		373,848
Adjustments due to new guidance | ASU 2014-09
ASSETS
Non-cash royalty receivable						8,900
Total current assets						8,900
Total assets						8,900
LIABILITIES AND SHAREHOLDERS’ EQUITY (DEFICIT)
Current portion of deferred revenue						41
Total current liabilities						41
Deferred revenue, net of current portion						(5,231)
Total liabilities						(5,190)
Shareholders' deficit:
Accumulated deficit						14,090
Total shareholders’ equity (deficit)						14,090
Total liabilities and shareholders’ equity (deficit)						8,900
Deferred tax assets:
Increase (decrease) in deferred tax assets						3,900
Milestone recognized as contract asset				$ 5,000
Potential milestone payment			$ 5,000		$ 5,000
Phase 1 clinical trial | Takeda
Deferred tax assets:
Milestone recognized as contract asset		$ 5,000
Phase 1 clinical trial | Adjustments due to new guidance | Takeda | ASU 2014-09
Deferred tax assets:
Potential milestone payment						$ 5,000