Commitments and Contingencies - Narrative - Environmental Reserves and Estimated Recoveries (Details) - USD ($)	6 Months Ended	12 Months Ended
	Jun. 30, 2018	Nov. 30, 2003	Jun. 30, 2017
Site Contingency [Line Items]
Non-reimbursable percentage of environmental costs	24.00%
Northrop
Site Contingency [Line Items]
Current annual billing limitation	$ 6,000,000
Reimbursable remediation cost	$ 189,700,000
Northrop | Maximum
Site Contingency [Line Items]
Reimbursable remediation cost			$ 189,700,000
Atlantic Research Corporation
Site Contingency [Line Items]
Pre-close environmental costs		$ 20,000,000.0
Various Environmental Matters
Site Contingency [Line Items]
Environmental remediation term	15 years