Balance Sheet Accounts and Supplemental Disclosures - Other Current Assets, net (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Deferred costs recoverable from the U.S. government	$ 41.0	$ 47.1
Income tax receivable	46.9	43.4
Prepaid expenses	18.0	13.9
Inventories	10.0	24.0
Other	20.6	17.4
Other current assets, net	$ 136.5	$ 145.8