Commitments and Contingencies - Summary of Environmental Reserve Activity (Details) $ in Millions	3 Months Ended
	Mar. 31, 2018 USD ($)
Accrual for Environmental Loss Contingencies [Roll Forward]
Beginning balance	$ 341.4
Additions	5.5
Expenditures	(8.5)
Ending balance	338.4
Aerojet Rocketdyne- Sacramento
Accrual for Environmental Loss Contingencies [Roll Forward]
Beginning balance	206.5
Additions	5.1
Expenditures	(3.1)
Ending balance	208.5
Aerojet Rocketdyne- BPOU
Accrual for Environmental Loss Contingencies [Roll Forward]
Beginning balance	116.4
Additions	0.4
Expenditures	(4.5)
Ending balance	112.3
Other Aerojet Rocketdyne Sites
Accrual for Environmental Loss Contingencies [Roll Forward]
Beginning balance	13.7
Additions	0.1
Expenditures	(0.7)
Ending balance	13.1
Total Aerojet Rocketdyne
Accrual for Environmental Loss Contingencies [Roll Forward]
Beginning balance	336.6
Additions	5.6
Expenditures	(8.3)
Ending balance	333.9
Other
Accrual for Environmental Loss Contingencies [Roll Forward]
Beginning balance	4.8
Additions	(0.1)
Expenditures	(0.2)
Ending balance	$ 4.5