Commitments and Contingencies - Summary of Environmental Reserve Activity (Details) - USD ($) $ in Millions	1 Months Ended	6 Months Ended
	Dec. 31, 2015	Jun. 30, 2016
Accrual for Environmental Loss Contingencies [Roll Forward]
Beginning balance	$ 306.1	$ 302.3
Additions	0.5	6.8
Expenditures	(4.3)	(17.4)
Ending balance	302.3	291.7
Aerojet Rocketdyne- Sacramento
Accrual for Environmental Loss Contingencies [Roll Forward]
Beginning balance	153.0	152.6
Additions	0.5	3.4
Expenditures	(0.9)	(9.2)
Ending balance	152.6	146.8
Aerojet Rocketdyne- BPOU
Accrual for Environmental Loss Contingencies [Roll Forward]
Beginning balance	140.1	136.7
Additions	0.0	2.2
Expenditures	(3.4)	(6.3)
Ending balance	136.7	132.6
Other Aerojet Rocketdyne Sites
Accrual for Environmental Loss Contingencies [Roll Forward]
Beginning balance	7.8	7.8
Additions	0.0	1.2
Expenditures	0.0	(1.4)
Ending balance	7.8	7.6
Total Aerojet Rocketdyne
Accrual for Environmental Loss Contingencies [Roll Forward]
Beginning balance	300.9	297.1
Additions	0.5	6.8
Expenditures	(4.3)	(16.9)
Ending balance	297.1	287.0
Other
Accrual for Environmental Loss Contingencies [Roll Forward]
Beginning balance	5.2	5.2
Additions	0.0	0.0
Expenditures	0.0	(0.5)
Ending balance	$ 5.2	$ 4.7