Balance Sheet Accounts and Supplemental Disclosures - Other Noncurrent Assets, net (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Real estate held for entitlement and leasing	$ 101.8	$ 100.3
Receivable from Northrop Grumman Corporation for environmental remediation costs	40.5	46.5
Deferred costs recoverable from the U.S. government	54.7	54.8
Other	57.8	54.0
Other noncurrent assets, net	$ 254.8	$ 255.6