Commitments and Contingencies - Research Collaboration and Licensing Agreements (Details) - Agreements with Royalty Provisions - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Research Collaboration and Licensing Arrangements [Line Items]
Royalty rate minimum	0.50%
Royalty rate maximum	15.00%
Royalty expense	$ 30.8	$ 125.3	$ 247.9