Retirement Benefits - Narrative - Summary of Components of Retirement Benefit Expense (Income) (Details)	12 Months Ended
Dec. 31, 2020
Retirement Benefits [Abstract]
Difference between expected return and actual return on market-related value of assets which smooths asset values (in years)	3 years

Difference Between Expected Return and Actual Return on Market-Related Value of Assets Which Stabilize Asset Values

No definition available.

Name:	ajrd_DifferenceBetweenExpectedReturnandActualReturnonMarketRelatedValueofAssetsWhichStabilizeAssetValues
Namespace Prefix:	ajrd_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration