Commitments and Contingencies - Environmental Reserves and Estimated Recoveries (Details) - USD ($)	9 Months Ended	12 Months Ended
	Sep. 30, 2017	Nov. 30, 2003	Dec. 31, 2017	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
Total reimbursable costs under the Northrop Agreement			$ 189,700,000
Northrop | Maximum
Site Contingency [Line Items]
Total reimbursable costs under the Northrop Agreement				$ 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]
Term For Contingent Consideration Arrangements	15 years