Commitments and Contingencies - Narrative - Environmental Reserves and Estimated Recoveries (Details) - USD ($)		3 Months Ended		12 Months Ended
	May 06, 2016	Sep. 30, 2018	Mar. 31, 2017	Dec. 31, 2020	Nov. 30, 2003	Jun. 30, 2017
Site Contingency [Line Items]
Environmental remediation cost expected period (years)				15 years
Estimated non-reimbursable costs (percentage)				12.00%
Northrop
Site Contingency [Line Items]
Recoverable percentage of costs (percentage)		88.00%		88.00%
Current annual billing limitation				$ 6,000,000.0
Total reimbursable costs under the Northrop Agreement				$ 189,700,000		$ 189,700,000
Accrual for environmental loss contingencies, revision in estimates		$ 43,000,000.0
Atlantic Research Corporation
Site Contingency [Line Items]
Pre-close environmental costs	$ 20,000,000.0		$ 20,000,000.0		$ 20,000,000.0