Related-Party Transactions (Details) $ in Millions	4 Months Ended	12 Months Ended
	May 07, 2018 member	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	May 31, 2016 member
Development Agreement
Inventory purchases | $		$ 0.3	$ 1.1	$ 0.8
Medtronic
Board of Directors' Appointment
Number of person allowed for member of entities Board of Directors	1
Number of voting member before the Series F preferred stock purchase agreement					1
Number of voting member after the Series F preferred stock purchase agreement					2