Commitments and Contingencies - Summary of Environmental Reserve Activity (Details) $ in Millions	3 Months Ended
	Mar. 31, 2017 USD ($)
Accrual for Environmental Loss Contingencies [Roll Forward]
Beginning balance	$ 349.7
Additions	4.7
Expenditures	(8.9)
Ending balance	345.5
Aerojet Rocketdyne- Sacramento
Accrual for Environmental Loss Contingencies [Roll Forward]
Beginning balance	210.1
Additions	2.9
Expenditures	(5.2)
Ending balance	207.8
Aerojet Rocketdyne- BPOU
Accrual for Environmental Loss Contingencies [Roll Forward]
Beginning balance	126.8
Additions	1.5
Expenditures	(3.2)
Ending balance	125.1
Other Aerojet Rocketdyne Sites
Accrual for Environmental Loss Contingencies [Roll Forward]
Beginning balance	8.5
Additions	0.1
Expenditures	(0.5)
Ending balance	8.1
Total Aerojet Rocketdyne
Accrual for Environmental Loss Contingencies [Roll Forward]
Beginning balance	345.4
Additions	4.5
Expenditures	(8.9)
Ending balance	341.0
Other
Accrual for Environmental Loss Contingencies [Roll Forward]
Beginning balance	4.3
Additions	0.2
Expenditures	0.0
Ending balance	$ 4.5