Summary of Significant Accounting Policies - Impact of Adopting ASC 606, Adjusted Balance Sheet (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	May 31, 2018	Jan. 02, 2018	Dec. 31, 2016	Jun. 30, 2016	Dec. 31, 2015	Jun. 30, 2015	Mar. 31, 2015
ASSETS
Cash and cash equivalents	$ 262,252,000	$ 267,107,000		$ 267,107,000	$ 159,964,000	$ 245,026,000		$ 278,109,000
Accounts receivable	1,701,000	2,649,000		2,649,000
Unbilled revenue	617,000	2,580,000		2,580,000
Non-cash royalty receivable	9,249,000			8,900,000
Inventory	0	1,038,000		1,038,000
Prepaid and other current assets	4,462,000	2,967,000		2,967,000
Total current assets	278,781,000	276,341,000		285,241,000
Property and equipment, net of accumulated depreciation	12,891,000	14,538,000		14,538,000
Other assets	3,709,000	3,797,000		3,797,000
Total assets	295,381,000	294,676,000		303,576,000
LIABILITIES AND SHAREHOLDERS' EQUITY (DEFICIT)
Accounts payable	11,365,000	8,562,000		8,562,000
Accrued compensation	11,796,000	11,473,000		11,473,000
Other accrued liabilities	20,465,000	15,767,000		15,767,000
Current portion of deferred lease incentive	837,000	784,000		784,000
Current portion of liability related to the sale of future royalties, net	25,880,000	17,779,000		17,779,000
Current portion of deferred revenue	317,000	1,405,000		1,446,000
Total current liabilities	70,660,000	55,770,000		55,811,000
Deferred lease incentive, net of current portion	4,675,000	5,129,000		5,129,000			$ 2,000,000
Deferred revenue, net of current portion	80,485,000	93,752,000		88,521,000
Convertible 4.5% senior notes, net	2,064,000	2,050,000		2,050,000
Liability related to the sale of future royalties, net	122,345,000	151,634,000		151,634,000
Other long-term liabilities	4,180,000	4,236,000		4,236,000
Total liabilities	284,409,000	312,571,000		307,381,000
Shareholders' deficit:
Preferred stock
Common stock	1,494,000	1,325,000		1,325,000
Additional paid-in capital	1,192,813,000	1,009,362,000		1,009,362,000
Accumulated deficit	(1,183,335,000)	(1,028,582,000)		(1,014,492,000)
Total shareholders' equity (deficit)	10,972,000	(17,895,000)		(3,805,000)	$ (152,850,000)	$ (82,304,000)		$ 35,104,000
Total liabilities and shareholders' equity (deficit)	295,381,000	294,676,000		303,576,000
Reduction to accumulated deficit related to previously delivered license	1,476,000
Deferred tax assets:
Increase (decrease) in valuation allowance	49,900,000
Milestone recognized as contract asset	500,000
Takeda
Shareholders' deficit:
Accumulated deficit				4,600,000
Deferred tax assets:
Potential milestone payment									$ 210,000,000
Pro forma as if previous accounting was in effect | ASU 2014-09
ASSETS
Cash and cash equivalents	262,252,000	267,107,000
Accounts receivable	1,701,000	2,649,000
Unbilled revenue	617,000	2,580,000
Inventory		1,038,000
Prepaid and other current assets	4,462,000	2,967,000
Total current assets	269,032,000	276,341,000
Property and equipment, net of accumulated depreciation	12,891,000	14,538,000
Other assets	3,709,000	3,797,000
Total assets	285,632,000	294,676,000
LIABILITIES AND SHAREHOLDERS' EQUITY (DEFICIT)
Accounts payable	11,365,000	8,562,000
Accrued compensation	11,796,000	11,473,000
Other accrued liabilities	20,465,000	15,767,000
Current portion of deferred lease incentive	837,000	784,000
Current portion of liability related to the sale of future royalties, net	25,880,000	17,779,000
Current portion of deferred revenue	297,000	1,405,000
Total current liabilities	70,640,000	55,770,000
Deferred lease incentive, net of current portion	4,675,000	5,129,000
Deferred revenue, net of current portion	83,710,000	93,752,000
Convertible 4.5% senior notes, net	2,064,000	2,050,000
Liability related to the sale of future royalties, net	122,345,000	151,634,000
Other long-term liabilities	4,180,000	4,236,000
Total liabilities	287,614,000	312,571,000
Shareholders' deficit:
Common stock	1,494,000	1,325,000
Additional paid-in capital	1,192,813,000	1,009,362,000
Accumulated deficit	(1,196,289,000)	(1,028,582,000)
Total shareholders' equity (deficit)	(1,982,000)	(17,895,000)
Total liabilities and shareholders' equity (deficit)	285,632,000	294,676,000
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
Phase 1 clinical trial | Novartis
Deferred tax assets:
Potential milestone payment	5,000,000
Phase 1 clinical trial | Takeda
Deferred tax assets:
Milestone earned, included in accounts receivable	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	$ 5,000,000
Technological Improvements
LIABILITIES AND SHAREHOLDERS' EQUITY (DEFICIT)
Deferred revenue, net of current portion	$ 5,900,000	$ 6,800,000
Technological Improvements | Takeda
Deferred tax assets:
Potential milestone payment				$ 400,000