Revenues - Narrative (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Revenue from External Customer [Line Items]
Fees to GPOs	$ 2,600	$ 2,100	$ 4,800	$ 4,000
Short-term deferred revenues	83,012		83,012		$ 81,835
Deferred revenues recognized	25,700		65,000
Contract with Customer, Liability, Current, Gross					92,900
Long-term deferred revenues	9,658		9,658		10,582
Deferred cost of sale,short-term
Revenue from External Customer [Line Items]
Short-term deferred revenues	$ 12,000		$ 12,000		$ 11,100