v3.25.2
Cover Page
 6 Months Ended
Jun. 30, 2025
Document Information [Line Items]  
Document Type S-4
Amendment Flag false
Entity Registrant Name Churchill Capital Corp IX/Cayman
Entity Central Index Key 0002006291
Entity Primary SIC Number 6770
Entity Incorporation, State or Country Code E9
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
Entity Filer Category Non-accelerated Filer
Entity Small Business true
Entity Emerging Growth Company true
Entity Ex Transition Period false
Business Contact [Member]  
Document Information [Line Items]  
Contact Personnel Name Jay Taragin
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