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