Revenue Arrangements - Bill and Melinda Gates Foundation (Narrative) (Details) - Bill and Melinda Gates Foundation - USD ($) $ in Millions |
12 Months Ended | ||||
|---|---|---|---|---|---|
Dec. 31, 2017 |
Dec. 31, 2016 |
Dec. 31, 2015 |
Dec. 31, 2014 |
Dec. 31, 2013 |
|
| Research and Development Arrangement, Contract to Perform for Others [Line Items] | |||||
| Aggregate amount of grant | $ 1.6 | $ 1.2 | |||
| Extended research term | 3 years | ||||
| Revenue recognized (less than) | $ 0.1 | $ 0.5 | $ 0.6 | ||