Summary of Significant Accounting Policies - Impact of Adopting ASC 606, Adjusted Balance Sheet (Details)		1 Months Ended	3 Months Ended	9 Months Ended	12 Months Ended
	Jan. 02, 2018 USD ($)	Oct. 31, 2018	Sep. 30, 2018 USD ($)	Sep. 30, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2010 USD ($)	Sep. 30, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
ASSETS
Cash and cash equivalents			$ 303,205,000	$ 303,205,000	$ 267,107,000		$ 194,851,000	$ 159,964,000
Accounts receivable			2,115,000	2,115,000	2,649,000
Unbilled revenue			521,000	521,000	2,580,000
Non-cash royalty receivable			8,115,000	8,115,000
Inventory			1,938,000	1,938,000	1,038,000
Prepaid and other current assets			6,320,000	6,320,000	2,967,000
Total current assets			322,714,000	322,714,000	276,341,000
Property and equipment, net of accumulated depreciation			13,209,000	13,209,000	14,538,000
Other assets			3,941,000	3,941,000	3,797,000
Total assets			339,864,000	339,864,000	294,676,000
LIABILITIES AND SHAREHOLDERS? EQUITY (DEFICIT)
Accounts payable			10,352,000	10,352,000	8,562,000
Accrued compensation			10,316,000	10,316,000	11,473,000
Other accrued liabilities			24,117,000	24,117,000	15,767,000
Current portion of deferred lease incentive			832,000	832,000	784,000
Current portion of liability related to the sale of future royalties, net			23,040,000	23,040,000	17,779,000
Current portion of deferred revenue			1,713,000	1,713,000	1,405,000
Total current liabilities			70,370,000	70,370,000	55,770,000
Deferred lease incentive, net of current portion			4,854,000	4,854,000	5,129,000				$ 2,000,000
Deferred revenue, net of current portion			80,592,000	80,592,000	93,752,000
Convertible 4.5% senior notes, net			2,060,000	2,060,000	2,050,000
Liability related to the sale of future royalties, net			130,907,000	130,907,000	151,634,000
Other long-term liabilities			4,193,000	4,193,000	4,236,000
Total liabilities			292,976,000	292,976,000	312,571,000
Shareholders' deficit:
Preferred stock
Common stock			1,490,000	1,490,000	1,325,000
Additional paid-in capital			1,186,934,000	1,186,934,000	1,009,362,000
Accumulated deficit			(1,141,536,000)	(1,141,536,000)	(1,028,582,000)
Total shareholders? equity (deficit)			46,888,000	46,888,000	(17,895,000)			$ (152,850,000)
Total liabilities and shareholders? equity (deficit)			339,864,000	339,864,000	294,676,000
Reduction to accumulated deficit related to previously delivered license			500,000	500,000
Deferred tax assets:
Milestone recognized as contract asset				500,000
Novartis
Deferred tax assets:
Potential milestone payment						$ 199,500,000
Takeda
Shareholders' deficit:
Reduction to accumulated deficit related to previously delivered license	$ 4,600,000
Amount allocated to undelivered rights to future technological improvements.	400,000
Pro forma as if previous accounting was in effect | ASU 2014-09
ASSETS
Cash and cash equivalents	267,107,000		303,205,000	303,205,000
Accounts receivable	2,649,000		2,115,000	2,115,000
Unbilled revenue	2,580,000		521,000	521,000
Non-cash royalty receivable	8,900,000
Inventory	1,038,000		1,938,000	1,938,000
Prepaid and other current assets	2,967,000		6,320,000	6,320,000
Total current assets	285,241,000		314,099,000	314,099,000
Property and equipment, net of accumulated depreciation	14,538,000		13,209,000	13,209,000
Other assets	3,797,000		3,941,000	3,941,000
Total assets	303,576,000		331,249,000	331,249,000
LIABILITIES AND SHAREHOLDERS? EQUITY (DEFICIT)
Accounts payable	8,562,000		10,352,000	10,352,000
Accrued compensation	11,473,000		10,316,000	10,316,000
Other accrued liabilities	15,767,000		24,117,000	24,117,000
Current portion of deferred lease incentive	784,000		832,000	832,000
Current portion of liability related to the sale of future royalties, net	17,779,000		23,040,000	23,040,000
Current portion of deferred revenue	1,446,000		1,455,000	1,455,000
Total current liabilities	55,811,000		70,112,000	70,112,000
Deferred lease incentive, net of current portion	5,129,000		4,854,000	4,854,000
Deferred revenue, net of current portion	88,521,000		83,834,000	83,834,000
Convertible 4.5% senior notes, net	2,050,000		2,060,000	2,060,000
Liability related to the sale of future royalties, net	151,634,000		130,907,000	130,907,000
Other long-term liabilities	4,236,000		4,193,000	4,193,000
Total liabilities	307,381,000		295,960,000	295,960,000
Shareholders' deficit:
Common stock	1,325,000		1,490,000	1,490,000
Additional paid-in capital	1,009,362,000		1,186,934,000	1,186,934,000
Accumulated deficit	(1,014,492,000)		(1,153,135,000)	(1,153,135,000)
Total shareholders? equity (deficit)	(3,805,000)		35,289,000	35,289,000
Total liabilities and shareholders? equity (deficit)	$ 303,576,000		331,249,000	331,249,000
Adjustments due to new guidance | ASU 2014-09
ASSETS
Non-cash royalty receivable					8,900,000
Total current assets					8,900,000
Total assets					8,900,000
LIABILITIES AND SHAREHOLDERS? EQUITY (DEFICIT)
Current portion of deferred revenue					41,000
Total current liabilities					41,000
Deferred revenue, net of current portion					(5,231,000)
Total liabilities					(5,190,000)
Shareholders' deficit:
Accumulated deficit					14,090,000
Total shareholders? equity (deficit)					14,090,000
Total liabilities and shareholders? equity (deficit)					8,900,000
Deferred tax assets:
Increase (decrease) in deferred tax assets					3,900,000
Milestone recognized as contract asset			500,000	5,000,000
Potential milestone payment			$ 500,000	$ 5,000,000
Phase 1 clinical trial | Adjustments due to new guidance | Takeda | ASU 2014-09
Deferred tax assets:
Potential milestone payment					$ 5,000,000
Subsequent event | Lilly
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Number of terminated development and commercialization licenses		3
Number of terminated pre-clinical stage programs		2
Number of terminated clinical stage program development and commercialization licenses		1