v3.26.1
Document and Entity Information - shares
3 Months Ended
Mar. 31, 2026
May 12, 2026
Cover [Abstract]    
Document Type 10-Q  
Amendment Flag false  
Document Period End Date Mar. 31, 2026  
Document Fiscal Year Focus 2026  
Document Fiscal Period Focus Q1  
Entity Registrant Name West Bay BDC LLC  
Entity Central Index Key 0002020354  
Entity Current Reporting Status Yes  
Entity Interactive Data Current Yes  
Current Fiscal Year End Date --12-31  
Entity Filer Category Non-accelerated Filer  
Entity Shell Company false  
Entity Small Business false  
Entity Emerging Growth Company true  
Securities Act File Number 814-01758  
Entity Incorporation, State or Country Code DE  
Entity Tax Identification Number 99-1657525  
Entity Address, Address Line One 200 West Street  
Entity Address, City or Town New York  
Entity Address, State or Province NY  
Entity Address, Postal Zip Code 10282  
City Area Code 312  
Local Phone Number 655-4419  
Document Quarterly Report true  
Document Transition Report false  
Entity Ex Transition Period true  
Entity Common Stock, Shares Outstanding   14,487,726