Commitments and Contingencies - Summary of Environmental Reserve Activity (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Accrual for Environmental Loss Contingencies [Roll Forward]
Beginning balance	$ 309.2	$ 327.9	$ 341.4
Additions	32.4	16.7	23.3
Expenditures	(41.0)	(35.4)	(36.8)
Ending balance	300.6	309.2	327.9
Aerojet Rocketdyne- Sacramento
Accrual for Environmental Loss Contingencies [Roll Forward]
Beginning balance	203.6	207.4	206.5
Additions	30.9	17.0	20.1
Expenditures	(26.1)	(20.8)	(19.2)
Ending balance	208.4	203.6	207.4
Aerojet Rocketdyne- BPOU
Accrual for Environmental Loss Contingencies [Roll Forward]
Beginning balance	89.6	103.8	116.4
Additions	(0.2)	(0.8)	2.3
Expenditures	(13.2)	(13.4)	(14.9)
Ending balance	76.2	89.6	103.8
Other Aerojet Rocketdyne Sites
Accrual for Environmental Loss Contingencies [Roll Forward]
Beginning balance	11.8	12.4	13.7
Additions	0.0	0.3	0.6
Expenditures	(1.2)	(0.9)	(1.9)
Ending balance	10.6	11.8	12.4
Total Aerojet Rocketdyne
Accrual for Environmental Loss Contingencies [Roll Forward]
Beginning balance	305.0	323.6	336.6
Additions	30.7	16.5	23.0
Expenditures	(40.5)	(35.1)	(36.0)
Ending balance	295.2	305.0	323.6
Other
Accrual for Environmental Loss Contingencies [Roll Forward]
Beginning balance	4.2	4.3	4.8
Additions	1.7	0.2	0.3
Expenditures	(0.5)	(0.3)	(0.8)
Ending balance	$ 5.4	$ 4.2	$ 4.3