Commitments and Contingencies - Environmental Matters (Details)			9 Months Ended	12 Months Ended
	May 09, 2017	Jun. 30, 2000 respondent	Sep. 30, 2017 LegalMatter	Dec. 31, 2002 USD ($) a	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Nov. 30, 2015 USD ($)	Nov. 30, 2014 USD ($)
Site Contingency [Line Items]
Accrued environmental costs					$ 341,400,000	$ 349,700,000	$ 302,300,000	$ 306,100,000	$ 166,000,000
Guarantee obligations					120,000,000.0
Various Environmental Matters
Site Contingency [Line Items]
Number of environmental remediation matters (over) | LegalMatter			40
Accrued environmental costs					341,400,000
Various Environmental Matters | Sacramento, California
Site Contingency [Line Items]
Accrued environmental costs					206,500,000
Various Environmental Matters | Sacramento, California | Environmental Protection Agency
Site Contingency [Line Items]
Non-contaminated land | a				2,600
Various Environmental Matters | Sacramento, California | Previously Reported
Site Contingency [Line Items]
Guarantee obligations				$ 20,000,000
Various Environmental Matters | Baldwin Park Operable Unit
Site Contingency [Line Items]
Accrued environmental costs					$ 116,400,000
Management agreement terms	10 years
Various Environmental Matters | Baldwin Park Operable Unit | Aerojet Rocketdyne Inc
Site Contingency [Line Items]
Percentage of responsibility in all project costs			74.00%
Various Environmental Matters | Baldwin Park Operable Unit | Environmental Protection Agency
Site Contingency [Line Items]
Number of other respondents | respondent		7
Various Environmental Matters | Contracting business
Site Contingency [Line Items]
Environmental obligation funding percent					99.00%
Various Environmental Matters | Minimum
Site Contingency [Line Items]
Estimated environmental costs					$ 341,400,000
Various Environmental Matters | Minimum | Sacramento, California
Site Contingency [Line Items]
Estimated environmental costs					206,500,000
Various Environmental Matters | Minimum | Baldwin Park Operable Unit
Site Contingency [Line Items]
Estimated environmental costs					116,400,000
Various Environmental Matters | Maximum
Site Contingency [Line Items]
Estimated environmental costs					503,400,000
Various Environmental Matters | Maximum | Sacramento, California
Site Contingency [Line Items]
Estimated environmental costs					325,200,000
Guarantee obligations				$ 75,000,000
Various Environmental Matters | Maximum | Baldwin Park Operable Unit
Site Contingency [Line Items]
Estimated environmental costs					$ 152,500,000