Summary of Significant Accounting Policies - Impact of Adopting ASC 606, Balance Sheet Adjustments (Details) $ in Thousands			3 Months Ended	12 Months Ended
	May 01, 2018 item	Jan. 02, 2018 USD ($)	Mar. 31, 2018 USD ($) item	Dec. 31, 2017 USD ($) item	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
ASSETS
Cash and cash equivalents			$ 218,383	$ 267,107	$ 126,568	$ 159,964
Accounts receivable			34	2,649
Unbilled revenue			2,838	2,580
Non-cash royalty receivable			16,090
Inventory			368	1,038
Prepaid and other current assets			5,680	2,967
Total current assets			247,434	276,341
Property and equipment, net of accumulated depreciation			12,850	14,538
Other assets			4,681	3,797
Total assets			264,965	294,676
LIABILITIES AND SHAREHOLDERS' DEFICIT
Accounts payable			7,786	8,562
Accrued compensation			5,671	11,473
Other accrued liabilities			23,756	15,767
Current portion of deferred lease incentive			784	784
Current portion of liability related to the sale of future royalties, net			27,619	17,779
Current portion of deferred revenue			611	1,405
Total current liabilities			66,227	55,770
Deferred lease incentive, net of current portion			4,933	5,129
Deferred revenue, net of current portion			81,522	93,752
Convertible 4.5% senior notes, net of deferred financing costs of $47 and $50, respectively			2,053	2,050
Liability related to the sale of future royalties, net			142,018	151,634
Other long-term liabilities			4,523	4,236
Total liabilities			301,276	312,571
Shareholders' deficit:
Preferred stock
Common stock			1,330	1,325
Additional paid-in capital			1,015,464	1,009,362
Accumulated deficit			(1,053,105)	(1,028,582)
Total shareholders’ deficit			(36,311)	(17,895)
Total liabilities and shareholders’ deficit			$ 264,965	$ 294,676
Reduction to accumulated deficit related to previously delivered license		$ 4,600
Novartis
Shareholders' deficit:
Number of Single Target Licenses | item			5	6
Number Of Licenses Terminated | item	1
Pro forma as if previous accounting was in effect | ASU 2014-09
ASSETS
Cash and cash equivalents		267,107	$ 218,383
Accounts receivable		2,649	34
Unbilled revenue		2,580	2,838
Non-cash royalty receivable		8,900	8,875
Inventory		1,038	368
Prepaid and other current assets		2,967	5,680
Total current assets		285,241	236,178
Property and equipment, net of accumulated depreciation		14,538	12,850
Other assets		3,797	4,681
Total assets		303,576	253,709
LIABILITIES AND SHAREHOLDERS' DEFICIT
Accounts payable		8,562	7,786
Accrued compensation		11,473	5,671
Other accrued liabilities		15,767	23,756
Current portion of deferred lease incentive		784	784
Current portion of liability related to the sale of future royalties, net		17,779	27,619
Current portion of deferred revenue		1,446	569
Total current liabilities		55,811	66,185
Deferred lease incentive, net of current portion		5,129	4,933
Deferred revenue, net of current portion		88,521	84,423
Convertible 4.5% senior notes, net of deferred financing costs of $47 and $50, respectively		2,050	2,053
Liability related to the sale of future royalties, net		151,634	142,018
Other long-term liabilities		4,236	4,523
Total liabilities		307,381	304,135
Shareholders' deficit:
Common stock		1,325	1,330
Additional paid-in capital		1,009,362	1,015,464
Accumulated deficit		(1,014,492)	(1,067,220)
Total shareholders’ deficit		(3,805)	(50,426)
Total liabilities and shareholders’ deficit		$ 303,576	$ 253,709
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' 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’ deficit				14,090
Total liabilities and shareholders’ deficit				$ 8,900