Commitments and Contingencies - Summary of Environmental Reserve Activity (Details) $ in Millions	3 Months Ended
	Mar. 31, 2020 USD ($)
Accrual for Environmental Loss Contingencies [Roll Forward]
Beginning balance	$ 309.2
Additions/Adjustments	1.3
Expenditures	(7.9)
Ending balance	302.6
Aerojet Rocketdyne- Sacramento
Accrual for Environmental Loss Contingencies [Roll Forward]
Beginning balance	203.6
Additions/Adjustments	4.1
Expenditures	(3.5)
Ending balance	204.2
Aerojet Rocketdyne- BPOU
Accrual for Environmental Loss Contingencies [Roll Forward]
Beginning balance	89.6
Additions/Adjustments	(0.8)
Expenditures	(4.0)
Ending balance	84.8
Other Aerojet Rocketdyne Sites
Accrual for Environmental Loss Contingencies [Roll Forward]
Beginning balance	11.8
Additions/Adjustments	(2.2)
Expenditures	(0.3)
Ending balance	9.3
Total Aerojet Rocketdyne
Accrual for Environmental Loss Contingencies [Roll Forward]
Beginning balance	305.0
Additions/Adjustments	1.1
Expenditures	(7.8)
Ending balance	298.3
Other
Accrual for Environmental Loss Contingencies [Roll Forward]
Beginning balance	4.2
Additions/Adjustments	0.2
Expenditures	(0.1)
Ending balance	$ 4.3