Commitments and Contingencies - Summary of Environmental Reserve Activity (Details) - USD ($) $ in Millions	1 Months Ended	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2016	Nov. 30, 2015	Nov. 30, 2014
Accrual for Environmental Loss Contingencies [Roll Forward]
Beginning balance	$ 306.1	$ 302.3	$ 166.0	$ 171.3
Additions	0.5	87.4	176.6	33.7
Expenditures	(4.3)	(40.0)	(36.5)	(39.0)
Ending balance	302.3	349.7	306.1	166.0
Aerojet Rocketdyne- Sacramento [Member]
Accrual for Environmental Loss Contingencies [Roll Forward]
Beginning balance	153.0	152.6	130.4	128.0
Additions	0.5	80.0	44.3	24.0
Expenditures	(0.9)	(22.5)	(21.7)	(21.6)
Ending balance	152.6	210.1	153.0	130.4
Aerojet Rocketdyne- BPOU [Member]
Accrual for Environmental Loss Contingencies [Roll Forward]
Beginning balance	140.1	136.7	21.7	26.9
Additions	0.0	3.5	129.7	4.5
Expenditures	(3.4)	(13.4)	(11.3)	(9.7)
Ending balance	136.7	126.8	140.1	21.7
Other Aerojet Rocketdyne Sites [Member]
Accrual for Environmental Loss Contingencies [Roll Forward]
Beginning balance	7.8	7.8	8.1	8.2
Additions	0.0	3.9	2.0	3.3
Expenditures	0.0	(3.2)	(2.3)	(3.4)
Ending balance	7.8	8.5	7.8	8.1
Total Aerojet Rocketdyne [Member]
Accrual for Environmental Loss Contingencies [Roll Forward]
Beginning balance	300.9	297.1	160.2	163.1
Additions	0.5	87.4	176.0	31.8
Expenditures	(4.3)	(39.1)	(35.3)	(34.7)
Ending balance	297.1	345.4	300.9	160.2
Other [Member]
Accrual for Environmental Loss Contingencies [Roll Forward]
Beginning balance	5.2	5.2	5.8	8.2
Additions	0.0	0.0	0.6	1.9
Expenditures	0.0	(0.9)	(1.2)	(4.3)
Ending balance	$ 5.2	$ 4.3	$ 5.2	$ 5.8