Basis of Presentation and Nature of Operations - AR1 Research and Development (Details) - U.S. Air Force - Liquid Booster Engine Development Project - USD ($) $ in Millions	3 Months Ended	45 Months Ended
Mar. 31, 2018	Sep. 30, 2018
U.S. Government Contractual Arrangements
Research and Development Arrangement, Contract to Perform for Others [Line Items]
AR1 research and development	$ 6.7	$ 56.1
AR1 R&D costs incurred		312.7
AR1 R&D costs net of reimbursements		88.2
Research and development expense		32.1
U.S. Government Contractual Arrangements \| U.S. Air Force
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Investment percentage of total funding required	66.67%
Customer funding to offset costs incurred, amount collected		(214.9)
U.S. Government Contractual Arrangements \| Aerojet Rocketdyne and United Launch Alliance
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Agreement value		804.0
U.S. Government Contractual Arrangements \| United Launch Alliance
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Customer funding to offset costs incurred, amount collected		$ (9.6)
U.S. Government Contractual Arrangements - Modified June 2018 \| U.S. Air Force
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Investment percentage of total funding required		83.33%
U.S. Government Contractual Arrangements - Modified June 2018 \| Aerojet Rocketdyne and United Launch Alliance
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Agreement value		$ 353.8

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