Commitments and Contingencies - Summary of Northrop Agreement Activity (Details) - Northrop $ in Millions	Jun. 30, 2018 USD ($)
Contingencies And Commitments [Line Items]
Total reimbursable costs under the Northrop Agreement	$ 189.7
Amount reimbursed to the Company through June 30, 2018	(128.2)
Potential future cost reimbursements available	$ 61.5