Commitments and Contingencies - Summary of Northrop Agreement Activity (Details) - Northrop $ in Millions	Dec. 31, 2017 USD ($)
Site Contingency [Line Items]
Total reimbursable costs under the Northrop Agreement	$ 189.7
Amount reimbursed through December 31, 2017	(125.2)
Receivable from Northrop included in the consolidated balance sheet as of December 31, 2017	$ 64.5

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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