Commitments and Contingencies - Summary of Environmental Reserve Activity (Details) $ in Millions	6 Months Ended
	Jun. 30, 2018 USD ($)
Accrual for Environmental Loss Contingencies [Roll Forward]
Beginning balance	$ 341.4
Additions	10.6
Expenditures	(18.3)
Ending balance	333.7
Aerojet Rocketdyne- Sacramento
Accrual for Environmental Loss Contingencies [Roll Forward]
Beginning balance	206.5
Additions	8.9
Expenditures	(10.0)
Ending balance	205.4
Aerojet Rocketdyne- BPOU
Accrual for Environmental Loss Contingencies [Roll Forward]
Beginning balance	116.4
Additions	0.8
Expenditures	(6.8)
Ending balance	110.4
Other Aerojet Rocketdyne Sites
Accrual for Environmental Loss Contingencies [Roll Forward]
Beginning balance	13.7
Additions	0.4
Expenditures	(1.0)
Ending balance	13.1
Total Aerojet Rocketdyne
Accrual for Environmental Loss Contingencies [Roll Forward]
Beginning balance	336.6
Additions	10.1
Expenditures	(17.8)
Ending balance	328.9
Other
Accrual for Environmental Loss Contingencies [Roll Forward]
Beginning balance	4.8
Additions	0.5
Expenditures	(0.5)
Ending balance	$ 4.8