Commitments and Contingencies - Summary of Environmental Reserve Activity (Details) - USD ($) $ in Millions	1 Months Ended	3 Months Ended
	Dec. 31, 2015	Mar. 31, 2016
Accrual for Environmental Loss Contingencies [Roll Forward]
Beginning balance	$ 306.1	$ 302.3
Additions	0.5	4.9
Expenditures	(4.3)	(7.9)
Ending balance	302.3	299.3
Aerojet Rocketdyne- Sacramento [Member]
Accrual for Environmental Loss Contingencies [Roll Forward]
Beginning balance	153.0	152.6
Additions	0.5	1.4
Expenditures	(0.9)	(3.9)
Ending balance	152.6	150.1
Aerojet Rocketdyne- BPOU [Member]
Accrual for Environmental Loss Contingencies [Roll Forward]
Beginning balance	140.1	136.7
Additions	0.0	2.6
Expenditures	(3.4)	(2.9)
Ending balance	136.7	136.4
Other Aerojet Rocketdyne Sites [Member]
Accrual for Environmental Loss Contingencies [Roll Forward]
Beginning balance	7.8	7.8
Additions	0.0	1.2
Expenditures	0.0	(0.7)
Ending balance	7.8	8.3
Total Aerojet Rocketdyne [Member]
Accrual for Environmental Loss Contingencies [Roll Forward]
Beginning balance	300.9	297.1
Additions	0.5	5.2
Expenditures	(4.3)	(7.5)
Ending balance	297.1	294.8
Other [Member]
Accrual for Environmental Loss Contingencies [Roll Forward]
Beginning balance	5.2	5.2
Additions	0.0	(0.3)
Expenditures	0.0	(0.4)
Ending balance	$ 5.2	$ 4.5