Balance Sheet Accounts and Supplemental Disclosures - Other Current Assets, net (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Recoverable from the U.S. government for acquisition integration costs	$ 10.9	$ 11.9
Recoverable from the U.S. government and other third parties for environmental remediation costs	25.6	25.2
Receivable from Northrop for environmental remediation costs	6.0	6.0
Prepaid expenses	19.2	16.5
Cost-share and other receivables, net	7.5	17.8
Recoverable from the U.S. government for competitive improvement program obligations	14.9	7.6
Income tax receivable	20.5	26.8
Other	5.2	11.1
Other current assets, net	$ 109.8	$ 122.9