v3.26.1
Cover - USD ($)
 10 Months Ended
Dec. 31, 2025
Mar. 30, 2026
Jun. 30, 2025
Document Type 10-K    
Amendment Flag false    
Document Annual Report true    
Document Transition Report false    
Document Period End Date Dec. 31, 2025    
Document Fiscal Period Focus FY    
Document Fiscal Year Focus 2025    
Current Fiscal Year End Date --12-31    
Entity File Number 001-42642    
Entity Registrant Name Perimeter Acquisition Corp. I    
Entity Central Index Key 0002061473    
Entity Tax Identification Number 98-1843886    
Entity Incorporation, State or Country Code E9    
Entity Address, Address Line One 4514 Cole Avenue    
Entity Address, Address Line Two Suite 1600    
Entity Address, City or Town Dallas    
Entity Address, State or Province TX    
Entity Address, Postal Zip Code 75204    
City Area Code (512)    
Local Phone Number 200-2533    
Entity Well-known Seasoned Issuer No    
Entity Voluntary Filers No    
Entity Current Reporting Status Yes    
Entity Interactive Data Current Yes    
Entity Filer Category Non-accelerated Filer    
Entity Small Business true    
Entity Emerging Growth Company true    
Elected Not To Use the Extended Transition Period false    
Entity Shell Company true    
Entity Public Float     $ 245,605,500
Documents Incorporated by Reference [Text Block] None    
Inception date Mar. 06, 2025    
ICFR Auditor Attestation Flag false    
Document Financial Statement Error Correction [Flag] false    
Auditor Opinion [Text Block] We have audited the accompanying balance sheet of Perimeter Acquisition Corp. I (the “Company”) as of December 31, 2025, the related statement of operations, changes in shareholders’ deficit, and cash flows for the period from March 6, 2025 (inception) through December 31, 2025, and the related notes (collectively referred to as the “financial statements”). In our opinion, the financial statements present fairly, in all material respects, the financial position of the Company as of December 31, 2025, and the results of its operations and its cash flows for the period from March 6, 2025 (inception) through December 31, 2025, in conformity with accounting principles generally accepted in the United States of America.    
Auditor Name WithumSmith+Brown, PC    
Auditor Location New York, New York    
Auditor Firm ID 100    
Units Each Consisting Of One Class Ordinary Share 0.0001 Par Value And One half Of One RedeemableWarrant [Member]      
Title of 12(b) Security Units, each consisting of one Class A ordinary share, $0.0001 par value, and one-half of one redeemable warrant    
Trading Symbol PMTRU    
Security Exchange Name NASDAQ    
Class A ordinary shares included as part of the units      
Title of 12(b) Security Class A ordinary shares included as part of the units    
Trading Symbol PMTR    
Security Exchange Name NASDAQ    
Redeemable warrants included as part of the units, each whole warrant exercisable for one Class A ordinary share at an exercise price of $11.50      
Title of 12(b) Security Redeemable warrants included as part of the units, each whole warrant exercisable for one Class A ordinary share at an exercise price of $11.50    
Trading Symbol PMTRW    
Security Exchange Name NASDAQ    
Common Class A [Member]      
Entity Common Stock, Shares Outstanding   24,788,000  
Entity Listing, Par Value Per Share $ 0.0001    
Common Class B [Member]      
Entity Common Stock, Shares Outstanding   6,037,500  
Entity Listing, Par Value Per Share $ 0.0001