Property and equipment, net - Narrative (Details) $ in Millions	1 Months Ended
	Oct. 31, 2017 USD ($) ft²	Mar. 31, 2017 USD ($)	Apr. 30, 2016 USD ($) ft²	Sep. 30, 2016 building agreement
Laboratory and Office Space in Wilmington, Delaware
Property and equipment, net
Square footage | ft²			190,000
Purchase price of land and office building(s)			$ 81.3
Laboratory and Office Space in Wilmington, Delaware | Property and Equipment
Property and equipment, net
Purchase price of land and office building(s)			45.9
Laboratory and Office Space in Wilmington, Delaware | Land
Property and equipment, net
Purchase price of land and office building(s)			3.8
Laboratory and Office Space in Wilmington, Delaware | Building And Leasehold Improvements
Property and equipment, net
Purchase price of land and office building(s)			$ 31.6
Laboratory and Office Space in Wilmington, Delaware | Building
Property and equipment, net
Estimated useful life			P40Y
Land and Two Buildings in Wilmington, Delaware | 1701 Augustine Cutoff, Wilmington, Delaware
Property and equipment, net
Purchase price of land and office building(s)		$ 8.1
Number of agreements | agreement				2
Number of buildings | building				2
Land and Two Buildings in Wilmington, Delaware | 1701 Augustine Cutoff, Wilmington, Delaware | Land
Property and equipment, net
Purchase price of land and office building(s)		1.2
Land and Two Buildings in Wilmington, Delaware | 1701 Augustine Cutoff, Wilmington, Delaware | Building And Leasehold Improvements
Property and equipment, net
Purchase price of land and office building(s)		$ 6.9
Office Building in Wilmington, Delaware | Building
Property and equipment, net
Square footage | ft²	154,000
Office building	$ 91.3
Estimated useful life	P40Y