Commitments and Contingencies - Other (Details) $ in Millions	12 Months Ended
	Dec. 31, 2015 USD ($)
Other Funding Commitments
Accrued expenses for clinical studies	$ 1.6
Maximum
Indemnification Obligations
Initial indemnification exposure for actions or events involving officers, directors and certain employees	1.5
Option Agreement with Velo Bio, LLC
Contingent Regulatory and Commercial Milestone Payments
Maximum payments to be made, upon the exercise of the option to acquire the global rights to the DIF program ("DIF Rights"), excluding sales milestone payments	65.0
Maximum sales milestone payments to be made based on the achievement of annual sales milestones at specified targets	250.0
Option Agreement with Velo Bio, LLC | Minimum
Contingent Regulatory and Commercial Milestone Payments
Annual sales milestone target	100.0
Option Agreement with Velo Bio, LLC | Maximum
Contingent Regulatory and Commercial Milestone Payments
Annual sales milestone target	$ 900.0