v3.26.1
Index 600 Stock Portfolio N-1A Cover
Dec. 31, 2025
Prospectus [Line Items]  
Document Type 485BPOS
Amendment Flag false
Registrant Name NORTHWESTERN MUTUAL SERIES FUND, INC.
Entity Central Index Key 0000742212
Entity Investment Company Type N-1A
Document Period End Date Dec. 31, 2025
Prospectus Date May 01, 2026