Commitments and Contingencies - Environmental Reserves and Estimated Recoveries (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Nov. 30, 2003
Site Contingency [Line Items]
Period of estimating environmental remediation costs		15 years
Pre-Close Environmental Costs		$ 20,000,000
Non-reimbursable percentage of environmental costs		24.00%
Northrop [Member]
Site Contingency [Line Items]
Current annual billing limitation		$ 6,000,000.0
Nontrade receivables		68,000,000
Total reimbursable costs under the Northrop Agreement		189,700,000
Northrop [Member] | Maximum [Member]
Site Contingency [Line Items]
Total reimbursable costs under the Northrop Agreement		$ 189,700,000
Atlantic Research Corporation [Member]
Site Contingency [Line Items]
Pre-Close Environmental Costs			$ 20,000,000.0
Atlantic Research Corporation [Member] | Forecast [Member]
Site Contingency [Line Items]
Pre-Close Environmental Costs	$ 20,000,000