Commitments and Contingencies - Summary of Northrop Agreement Activity (Details) - Northrop - USD ($) $ in Millions	Dec. 31, 2020	Jun. 30, 2017
Site Contingency [Line Items]
Total reimbursable costs under the Northrop Agreement	$ 189.7	$ 189.7
Amount reimbursed to the Company through December 31, 2020	(143.2)
Receivable from Northrop included in the balance sheet at December 31, 2020	$ 46.5

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