v3.26.1
Shareholder Report
6 Months Ended
Jun. 30, 2026
USD ($)
Holding
Shareholder Report [Line Items]  
Document Type N-CSRS
Amendment Flag false
Registrant Name Core Bond Portfolio
Entity Central Index Key 0001104129
Entity Investment Company Type N-1A
Document Period End Date Jun. 30, 2026
Shareholder Report Annual or Semi-Annual Semi-Annual Shareholder Report
C000014262  
Shareholder Report [Line Items]  
Fund Name Core Bond Portfolio
No Trading Symbol [Flag] true
Annual or Semi-Annual Statement [Text Block] This semi-annual shareholder report contains important information about the Core Bond Portfolio (the "Fund") for the period of January 1, 2026 to June 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 Core Bond 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
Core Bond Portfolio
$23
0.47%Footnote Reference1
Footnote Description
Footnote1
Annualized
Expenses Paid, Amount $ 23
Expense Ratio, Percent 0.47% [1]
AssetsNet $ 700,764,330
Holdings Count | Holding 377
InvestmentCompanyPortfolioTurnover 146.00%
Additional Fund Statistics [Text Block]

Key Fund Statistics

Table Summary
Total Net Assets
$700,764,330
# of Portfolio Holdings
377
Portfolio Turnover Rate
146%
Holdings [Text Block]

Asset Allocation (% of total investments)

Credit Rating Chart
Table Summary
Value
Value
OtherFootnote Reference*
0.6%
Exchange-Traded Funds
2.7%
Short-Term Investments
4.8%
Collateralized Mortgage Obligations
5.0%
Commercial Mortgage-Backed Securities
7.7%
Asset-Backed Securities
13.4%
U.S. Treasury Obligations
16.5%
Corporate Bonds
24.2%
U.S. Government Agency Mortgage-Backed Securities
25.1%
Footnote Description
Footnote*
Investment types less than 1% each

Credit Quality (% of net assets)Footnote Referencea

Group By Asset Type Chart
Table Summary
Value
Value
Cash & Equivalents
(2.7%)
Not Rated
3.8%
CCC or Lower
0.2%
B
0.9%
BB
2.5%
BBB
14.4%
A
13.7%
AA
54.1%
AAA
13.1%
Footnote Description
Footnotea
Ratings are based on Moody’s Investors Service, Inc. (“Moody’s”), S&P Global Ratings (“S&P”) or Fitch Ratings (“Fitch”), or Kroll Bond Rating Agency, LLC (“Kroll”) for securitized debt instruments only (such as asset-backed and mortgage-backed securities). If securities are rated differently by the ratings agencies, the highest rating is applied. Moody's ratings are converted to the S&P, Fitch and Kroll scale with ratings ranging 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 the national ratings agencies stated above.
Material Fund Change [Text Block]
Updated Prospectus Phone Number 1-800-262-1122
[1] Annualized