| Document and Entity Information | Jun. 20, 2024 |
| Document And Entity Information [Line Items] | |
| Entity Registrant Name | Churchill Capital Corp IX/Cayman |
| Amendment Flag | false |
| Entity Central Index Key | 0002006291 |
| Document Type | 8-K |
| Document Period End Date | Jun. 20, 2024 |
| Entity Incorporation State Country Code | E9 |
| Entity File Number | 001-42041 |
| Entity Tax Identification Number | 86-1885237 |
| Entity Address, Address Line One | 640 Fifth Avenue |
| Entity Address, Address Line Two | 14th Floor |
| Entity Address, City or Town | New York |
| Entity Address, State or Province | NY |
| Entity Address, Postal Zip Code | 10019 |
| City Area Code | (212) |
| Local Phone Number | 380-7500 |
| Written Communications | false |
| Soliciting Material | false |
| Pre Commencement Tender Offer | false |
| Pre Commencement Issuer Tender Offer | false |
| Entity Emerging Growth Company | true |
| Entity Ex Transition Period | false |
| Units Each Consisting Of One Class A Ordinary Share And One Quarter Of One Redeemable Warrant [Member] | |
| Document And Entity Information [Line Items] | |
| Security 12b Title | Units, each consisting of one Class A ordinary share and one-quarter of one redeemable warrant |
| Trading Symbol | CCIXU |
| Security Exchange Name | NASDAQ |
| Common Stock [Member] | |
| Document And Entity Information [Line Items] | |
| Security 12b Title | Class A ordinary shares, par value $0.0001 per share |
| Trading Symbol | CCIX |
| Security Exchange Name | NASDAQ |
| Warrants Each Whole Warrant Exercisable For One Class A Ordinary Share At An Exercise Price Of 11.50 Per Share [Member] | |
| Document And Entity Information [Line Items] | |
| Security 12b Title | Warrants, each whole warrant exercisable for one Class A ordinary share at an exercise price of $11.50 per share |
| Trading Symbol | CCIXW |
| Security Exchange Name | NASDAQ |
