Basis of Presentation and Accounting Policies - Narrative (Details) $ in Thousands	3 Months Ended	9 Months Ended
	Sep. 30, 2018 USD ($)	Sep. 30, 2018 USD ($) segment	Jan. 01, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Number of operating segments | segment		1
Cumulative effect adjustment to accumulated deficit				$ 9,229	$ 0
Deferred revenue	$ 39,770	$ 39,770		1,654
Other income (expense), net	61,200	(88,200)
Deferred tax asset				1,600,000
Deferred tax assets, valuation allowance	204,700	204,700
HIV Protease Inhibitor Royalty
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Deferred revenue				6,900
Accumulated Deficit
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect adjustment to accumulated deficit				33,349	$ (9,371)
Accounting Standards Update 2014-09
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect adjustment to accumulated deficit	$ (24,120)	$ (24,120)
Accounting Standards Update 2014-09 | Accumulated Deficit
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect adjustment to accumulated deficit			$ 8,300
Accounting Standards Update 2016-01 | Accumulated Deficit
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect adjustment to accumulated deficit				$ 25,100
Accounting Standards Update 2016-16
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Deferred tax asset			204,700
Deferred tax assets, valuation allowance			$ 204,700