Commitments and Contingencies - Summary of Environmental Reserve Activity (Details) $ in Millions	6 Months Ended
	Jun. 30, 2020 USD ($)
Accrual for Environmental Loss Contingencies [Roll Forward]
Beginning balance	$ 309.2
Additions/Adjustments	6.5
Expenditures	(16.4)
Ending balance	299.3
Aerojet Rocketdyne- Sacramento
Accrual for Environmental Loss Contingencies [Roll Forward]
Beginning balance	203.6
Additions/Adjustments	9.2
Expenditures	(8.7)
Ending balance	204.1
Aerojet Rocketdyne- BPOU
Accrual for Environmental Loss Contingencies [Roll Forward]
Beginning balance	89.6
Additions/Adjustments	(0.9)
Expenditures	(6.8)
Ending balance	81.9
Other Aerojet Rocketdyne Sites
Accrual for Environmental Loss Contingencies [Roll Forward]
Beginning balance	11.8
Additions/Adjustments	(2.0)
Expenditures	(0.7)
Ending balance	9.1
Total Aerojet Rocketdyne
Accrual for Environmental Loss Contingencies [Roll Forward]
Beginning balance	305.0
Additions/Adjustments	6.3
Expenditures	(16.2)
Ending balance	295.1
Other
Accrual for Environmental Loss Contingencies [Roll Forward]
Beginning balance	4.2
Additions/Adjustments	0.2
Expenditures	(0.2)
Ending balance	$ 4.2