Commitments and Contingencies - Summary of Environmental Reserve Activity (Details) $ in Millions	9 Months Ended
	Sep. 30, 2020 USD ($)
Accrual for Environmental Loss Contingencies [Roll Forward]
Beginning balance	$ 309.2
Additions/Adjustments	23.3
Expenditures	(28.4)
Ending balance	304.1
Aerojet Rocketdyne- Sacramento
Accrual for Environmental Loss Contingencies [Roll Forward]
Beginning balance	203.6
Additions/Adjustments	22.8
Expenditures	(17.4)
Ending balance	209.0
Aerojet Rocketdyne- BPOU
Accrual for Environmental Loss Contingencies [Roll Forward]
Beginning balance	89.6
Additions/Adjustments	(0.7)
Expenditures	(9.6)
Ending balance	79.3
Other Aerojet Rocketdyne Sites
Accrual for Environmental Loss Contingencies [Roll Forward]
Beginning balance	11.8
Additions/Adjustments	(0.4)
Expenditures	(1.1)
Ending balance	10.3
Total Aerojet Rocketdyne
Accrual for Environmental Loss Contingencies [Roll Forward]
Beginning balance	305.0
Additions/Adjustments	21.7
Expenditures	(28.1)
Ending balance	298.6
Other
Accrual for Environmental Loss Contingencies [Roll Forward]
Beginning balance	4.2
Additions/Adjustments	1.6
Expenditures	(0.3)
Ending balance	$ 5.5