Revenue Recognition - Narrative (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2018	Jan. 01, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenue, payment term	30 days
Credits to revenue as a percentage of gross product revenues (or less)	1.00%
Early access sales accrual	$ 290,457	$ 225,128	$ 232,401
Cumulative effect adjustment for adoption of new accounting guidance			9,229	$ 0
Accumulated Deficit
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Cumulative effect adjustment for adoption of new accounting guidance			$ 33,349	$ (9,371)
Accounting Standards Update 2014-09
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Cumulative effect adjustment for adoption of new accounting guidance	$ (24,120)
Accounting Standards Update 2014-09 | Accumulated Deficit
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Cumulative effect adjustment for adoption of new accounting guidance		$ 8,300