Revenue recognition - Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Disaggregation of Revenue [Line Items]
Total revenues	$ 190.6	$ 117.8	$ 117.8
U.S Government
Disaggregation of Revenue [Line Items]
Total revenues	93.7		80.8
Non-U.S. Government
Disaggregation of Revenue [Line Items]
Total revenues	96.9		37.0
Product sales, net
Disaggregation of Revenue [Line Items]
Total revenues	153.0	75.8	75.8
Product sales, net | U.S Government
Disaggregation of Revenue [Line Items]
Total revenues	73.3		66.0
Product sales, net | Non-U.S. Government
Disaggregation of Revenue [Line Items]
Total revenues	79.7		9.8
Contract manufacturing
Disaggregation of Revenue [Line Items]
Total revenues	15.9	26.1	26.1
Contract manufacturing | U.S Government
Disaggregation of Revenue [Line Items]
Total revenues	0.0		0.0
Contract manufacturing | Non-U.S. Government
Disaggregation of Revenue [Line Items]
Total revenues	15.9		26.1
Contracts and grants
Disaggregation of Revenue [Line Items]
Total revenues	21.7	$ 15.9	15.9
Contracts and grants | U.S Government
Disaggregation of Revenue [Line Items]
Total revenues	20.4		14.8
Contracts and grants | Non-U.S. Government
Disaggregation of Revenue [Line Items]
Total revenues	$ 1.3		$ 1.1