Summary of Significant Accounting Policies - Impact of Adopting ASC 606, Pro Forma Balance Sheet (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Jan. 02, 2018	Dec. 31, 2017	Sep. 30, 2017	Dec. 31, 2016	Dec. 31, 2015
ASSETS
Cash and cash equivalents	$ 303,205		$ 267,107	$ 194,851	$ 159,964
Accounts receivable	2,115		2,649
Unbilled revenue	521		2,580
Contract asset	500
Non-cash royalty receivable	8,115
Inventory	1,938		1,038
Prepaid and other current assets	6,320		2,967
Total current assets	322,714		276,341
Property and equipment, net of accumulated depreciation	13,209		14,538
Other assets	3,941		3,797
Total assets	339,864		294,676
LIABILITIES AND SHAREHOLDERS? EQUITY (DEFICIT)
Accounts payable	10,352		8,562
Accrued compensation	10,316		11,473
Other accrued liabilities	24,117		15,767
Current portion of deferred lease incentive	832		784
Current portion of liability related to the sale of future royalties, net	23,040		17,779
Current portion of deferred revenue	1,713		1,405
Total current liabilities	70,370		55,770
Deferred lease incentive, net of current portion	4,854		5,129			$ 2,000
Deferred revenue, net of current portion	80,592		93,752
Convertible 4.5% senior notes, net	2,060		2,050
Liability related to the sale of future royalties, net	130,907		151,634
Other long-term liabilities	4,193		4,236
Total liabilities	292,976		312,571
Shareholders' deficit:
Preferred stock
Common stock	1,490		1,325
Additional paid-in capital	1,186,934		1,009,362
Accumulated deficit	(1,141,536)		(1,028,582)
Total shareholders? equity (deficit)	46,888		(17,895)		$ (152,850)
Total liabilities and shareholders? equity (deficit)	339,864		$ 294,676
Pro forma as if previous accounting was in effect | ASU 2014-09
ASSETS
Cash and cash equivalents	303,205	$ 267,107
Accounts receivable	2,115	2,649
Unbilled revenue	521	2,580
Non-cash royalty receivable		8,900
Inventory	1,938	1,038
Prepaid and other current assets	6,320	2,967
Total current assets	314,099	285,241
Property and equipment, net of accumulated depreciation	13,209	14,538
Other assets	3,941	3,797
Total assets	331,249	303,576
LIABILITIES AND SHAREHOLDERS? EQUITY (DEFICIT)
Accounts payable	10,352	8,562
Accrued compensation	10,316	11,473
Other accrued liabilities	24,117	15,767
Current portion of deferred lease incentive	832	784
Current portion of liability related to the sale of future royalties, net	23,040	17,779
Current portion of deferred revenue	1,455	1,446
Total current liabilities	70,112	55,811
Deferred lease incentive, net of current portion	4,854	5,129
Deferred revenue, net of current portion	83,834	88,521
Convertible 4.5% senior notes, net	2,060	2,050
Liability related to the sale of future royalties, net	130,907	151,634
Other long-term liabilities	4,193	4,236
Total liabilities	295,960	307,381
Shareholders' deficit:
Common stock	1,490	1,325
Additional paid-in capital	1,186,934	1,009,362
Accumulated deficit	(1,153,135)	(1,014,492)
Total shareholders? equity (deficit)	35,289	(3,805)
Total liabilities and shareholders? equity (deficit)	$ 331,249	$ 303,576