Summary of Significant Accounting Policies - Revenue (Detail) - USD ($) $ in Millions	1 Months Ended		9 Months Ended
	May 31, 2020	Mar. 31, 2020	Sep. 30, 2020
CEPI
Revenue Recognition
Funding from CEPI	$ 399.5	$ 3.9
Amount of CEPI Grant Funding	257.0
Amount of CEPI Forgivable Loan Funding	$ 142.5
Government contract
Revenue Recognition
Maximum funding entitled to receive from U.S. government			$ 1,660.0
Amount of transaction price not yet satisfied			1,620.0
Contract asset			3.8
Contract liability			26.5
Grant and other | CEPI
Revenue Recognition
Funding from CEPI			399.5
Amount of CEPI Grant Funding			257.0
Amount of CEPI Forgivable Loan Funding			$ 142.5