Balance Sheet Accounts and Supplemental Disclosures - Other Noncurrent Assets, net (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Real estate held for entitlement and leasing	$ 94.0	$ 91.8
Receivable from Northrop for environmental remediation costs	58.5	62.0
Recoverable from the U.S. government for acquisition integration costs	0.0	10.9
Recoverable from the U.S. government for conditional asset retirement obligations	22.7	20.3
Recoverable from the U.S. government for competitive improvement program obligations	18.4	1.3
Recoverable from the U.S. government for restructuring costs	25.5	12.8
Grantor trust	24.2	16.6
Note receivable, net	9.0	9.0
Income tax receivable	0.0	10.8
Other	7.0	8.5
Other noncurrent assets, net	$ 259.3	$ 244.0