Commitments and Contingencies - Summary of Environmental Reserve Activity (Details) - USD ($) $ in Millions	1 Months Ended	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2017	Dec. 31, 2016	Nov. 30, 2015
Accrual for Environmental Loss Contingencies [Roll Forward]
Beginning balance	$ 306.1	$ 349.7	$ 302.3	$ 166.0
Additions	0.5	31.3	87.4	176.6
Expenditures	(4.3)	(39.6)	(40.0)	(36.5)
Ending balance	302.3	341.4	349.7	306.1
Aerojet Rocketdyne- Sacramento
Accrual for Environmental Loss Contingencies [Roll Forward]
Beginning balance	153.0	210.1	152.6	130.4
Additions	0.5	19.2	80.0	44.3
Expenditures	(0.9)	(22.8)	(22.5)	(21.7)
Ending balance	152.6	206.5	210.1	153.0
Aerojet Rocketdyne- BPOU
Accrual for Environmental Loss Contingencies [Roll Forward]
Beginning balance	140.1	126.8	136.7	21.7
Additions	0.0	3.3	3.5	129.7
Expenditures	(3.4)	(13.7)	(13.4)	(11.3)
Ending balance	136.7	116.4	126.8	140.1
Other Aerojet Rocketdyne Sites
Accrual for Environmental Loss Contingencies [Roll Forward]
Beginning balance	7.8	8.5	7.8	8.1
Additions	0.0	8.0	3.9	2.0
Expenditures	0.0	(2.8)	(3.2)	(2.3)
Ending balance	7.8	13.7	8.5	7.8
Total Aerojet Rocketdyne
Accrual for Environmental Loss Contingencies [Roll Forward]
Beginning balance	300.9	345.4	297.1	160.2
Additions	0.5	30.5	87.4	176.0
Expenditures	(4.3)	(39.3)	(39.1)	(35.3)
Ending balance	297.1	336.6	345.4	300.9
Other
Accrual for Environmental Loss Contingencies [Roll Forward]
Beginning balance	5.2	4.3	5.2	5.8
Additions	0.0	0.8	0.0	0.6
Expenditures	0.0	(0.3)	(0.9)	(1.2)
Ending balance	$ 5.2	$ 4.8	$ 4.3	$ 5.2