Balance Sheet Accounts - Schedule of Other Current Assets, net (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Deferred costs recoverable from the U.S. government	$ 49.0	$ 51.4
Inventories	18.0	19.3
Receivable from Northrop Grumman Corporation (Northrop) for environmental remediation costs	6.0	6.0
Prepaid expenses	31.3	19.2
Cost-share and other receivables, net	23.2	7.5
Income taxes receivable	20.5	20.5
Other	8.7	5.2
Other current assets, net	$ 156.7	129.1
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Reclassification of inventories to contract assets		0.0
Contract asset
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Reclassification of inventories to contract assets		$ 117.1