v3.26.1
Shareholder Report
 6 Months Ended
Apr. 30, 2026
USD ($)
Holding
Shareholder Report [Line Items]  
Document Type N-CSRS
Amendment Flag false
Registrant Name SENIOR DEBT PORTFOLIO
Entity Central Index Key 0000933188
Entity Investment Company Type N-1A
Document Period End Date Apr. 30, 2026
Shareholder Report Annual or Semi-Annual Semi-Annual Shareholder Report
C000064522  
Shareholder Report [Line Items]  
Fund Name Senior Debt Portfolio
No Trading Symbol [Flag] true
Annual or Semi-Annual Statement [Text Block] This semi-annual shareholder report contains important information about the Senior Debt Portfolio (the "Fund") for the period of November 1, 2025 to April 30, 2026.
Additional Information [Text Block] You can find additional information about the Fund at www.eatonvance.com/open-end-mutual-fund-documents.php and selecting Eaton Vance Floating-Rate Advantage Fund. You can also request this information by contacting us at 1-800-262-1122.
Additional Information Phone Number 1-800-262-1122
Additional Information Website <span style="box-sizing: border-box; color: rgb(0, 0, 0); display: inline; flex-wrap: nowrap; font-size: 10.6667px; font-weight: 400; grid-area: auto; line-height: 16px; margin: 0px; overflow: visible; text-align: left; text-align-last: auto; white-space-collapse: preserve-breaks;">www.eatonvance.com/open-end-mutual-fund-documents.php</span>
Expenses [Text Block]

What were the Fund costs for the last six months?

(based on a hypothetical $10,000 investment)

Table Summary
Fund Name
Costs of a $10,000 investment
Costs paid as a percentage of a $10,000 investment
Senior Debt Portfolio
$107
2.16%Footnote Reference1
Footnote Description
Footnote1
 Annualized
Expenses Paid, Amount $ 107
Expense Ratio, Percent 2.16% [1]
AssetsNet $ 4,647,450,717
Holdings Count | Holding 618
InvestmentCompanyPortfolioTurnover 15.00%
Additional Fund Statistics [Text Block]

Key Fund Statistics

Table Summary
Total Net Assets
$4,647,450,717
# of Portfolio Holdings
618
Portfolio Turnover Rate
15%
Holdings [Text Block]

Asset Allocation (% of total investments)

Credit Rating Chart
Table Summary
Value
Value
OtherFootnote Reference†
1.1%
Common Stocks
1.1%
Short-Term Investments
2.5%
Asset-Backed Securities
2.8%
Corporate Bonds
5.0%
Senior Floating-Rate Loans
87.5%
Footnote Description
Footnote†
 Investment types less than 1% each

Credit Quality (% of net assets plus borrowings)Footnote Referencea

Group By Asset Type Chart
Table Summary
Value
Value
Not Rated
3.2%
CCC or Lower
3.1%
B
67.6%
BB
19.8%
BBB
3.3%
Footnote Description
Footnotea
 Excludes Short-Term Investments, Equities, Closed-End Funds and Exchange-Traded Funds. Ratings are categorized using S&P Global Ratings (“S&P”). Ratings range from AAA, being the highest, to D, being the lowest. Ratings of BBB or higher are considered to be investment-grade quality. Holdings designated as “Not Rated” (if any) are not rated by S&P.
Material Fund Change [Text Block]
Updated Prospectus Phone Number 1-800-262-1122
[1] Annualized