Commitments and Contingencies - Summary of Environmental Reserve Activity (Details) $ in Millions	9 Months Ended
	Sep. 30, 2021 USD ($)
Accrual for Environmental Loss Contingencies [Roll Forward]
Beginning balance	$ 300.6
Additions/Adjustments	30.0
Expenditures	(27.3)
Ending balance	303.3
Aerojet Rocketdyne- Sacramento
Accrual for Environmental Loss Contingencies [Roll Forward]
Beginning balance	208.4
Additions/Adjustments	24.2
Expenditures	(15.8)
Ending balance	216.8
Aerojet Rocketdyne- BPOU
Accrual for Environmental Loss Contingencies [Roll Forward]
Beginning balance	76.2
Additions/Adjustments	2.6
Expenditures	(9.4)
Ending balance	69.4
Other Aerojet Rocketdyne Sites
Accrual for Environmental Loss Contingencies [Roll Forward]
Beginning balance	10.6
Additions/Adjustments	3.1
Expenditures	(1.7)
Ending balance	12.0
Total Aerojet Rocketdyne
Accrual for Environmental Loss Contingencies [Roll Forward]
Beginning balance	295.2
Additions/Adjustments	29.9
Expenditures	(26.9)
Ending balance	298.2
Other
Accrual for Environmental Loss Contingencies [Roll Forward]
Beginning balance	5.4
Additions/Adjustments	0.1
Expenditures	(0.4)
Ending balance	$ 5.1