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N-2 - USD ($)
 6 Months Ended 12 Months Ended
Dec. 31, 2024
Jun. 30, 2024
Jun. 30, 2023
Jun. 30, 2022
Jun. 30, 2021
Jun. 30, 2020
Cover [Abstract]            
Entity Central Index Key 0001070732          
Amendment Flag false          
Document Type N-CSRS          
Entity Registrant Name Eaton Vance Senior Income Trust          
Financial Highlights [Abstract]            
Senior Securities [Table Text Block]
  Six Months Ended
December 31, 2024
(Unaudited) 		Year Ended June 30,
  2024 2023 2022 2021 2020
Ratios/Supplemental Data            
Net assets applicable to common shares, end of period (000’s omitted) $111,154 $107,136 $107,079 $106,208 $261,425 $234,657
Ratios (as a percentage of average daily net assets applicable to common shares):(6)†            
Expenses excluding interest and fees 1.88%(7)(8) 2.00%(8) 1.99%(8) 1.91% 1.96% 1.73%
Interest and fee expense(9) 1.00%(7) 1.21% 1.05% 0.47% 0.57% 1.19%
Total expenses 2.88%(7)(8) 3.21%(8) 3.04%(8) 2.38% 2.53% 2.92%
Net investment income 11.46%(7) 12.72% 11.15% 5.31% 6.08% 5.93%
Portfolio Turnover 12%(4) 34% 27% 43% 40% 57%
Senior Securities:            
Total notes payable outstanding (in 000’s) $21,000 $12,000 $20,000 $26,000 $103,000 $95,000
Asset coverage per $1,000 of notes payable(10) $8,085 $13,064 $8,235 $6,531 $3,903 $3,866
Total preferred shares outstanding 1,504 1,504 1,504 1,504 1,504 1,504
Asset coverage per preferred share(11) $72,434 $79,014 $71,481 $66,752 $71,484 $69,242
Involuntary liquidation preference per preferred share(12) $25,000 $25,000 $25,000 $25,000 $25,000 $25,000
Approximate market value per preferred share(12) $25,000 $25,000 $25,000 $25,000 $25,000 $25,000
(1) Computed using average common shares outstanding.
(2) Amount is less than $(0.005).
(3) Returns are historical and are calculated by determining the percentage change in net asset value or market value with all distributions reinvested. Distributions are assumed to be reinvested at prices obtained under the Trust’s dividend reinvestment plan.
(4) Not annualized.
(5) The total return based on net asset value reflects the impact of the tender and repurchase by the Trust of a portion of its common shares at 99% of the Trust’s net asset value per common share. Absent this transaction, the total return based on net asset value would have been (7.90)%.
(6) Ratios do not reflect the effect of dividend payments to preferred shareholders.
(7) Annualized.
(8) Includes a reduction by the investment adviser of a portion of its adviser fee due to the Trust’s investment in the Liquidity Fund (equal to less than 0.02%, less than 0.01% and less than 0.005% of average daily net assets for the six months ended December 31, 2024 and the years ended June 30, 2024 and 2023, respectively).
(9) Interest and fee expense relates to the notes payable to partially redeem the Trust’s Auction Preferred Shares and/or to fund investments (see Note 8).
(10) Calculated by subtracting the Trust’s total liabilities (not including the notes payable and preferred shares) from the Trust’s total assets, and dividing the result by the notes payable balance in thousands.
(11) Calculated by subtracting the Trust’s total liabilities (not including the notes payable and preferred shares) from the Trust’s total assets, dividing the result by the sum of the value of the notes payable and liquidation value of the preferred shares, and multiplying the result by the liquidation value of one preferred share.
(12) Plus accumulated and unpaid dividends.
* A portion of the distributions may be deemed a tax return of capital at year-end. See Note 3.
† Ratios based on net assets applicable to common shares plus preferred shares and borrowings are presented below. Ratios do not reflect the effect of dividend payments to preferred shareholders. Ratios for periods less than one year are annualized.
  Six Months Ended
December 31, 2024
(Unaudited) 		Year Ended June 30,
  2024 2023 2022 2021 2020
Expenses excluding interest and fees     1.24%   1.30%   1.29%    1.28%   1.25%   1.11%
Interest and fee expense     0.67%   0.78%   0.68%    0.32%   0.36%   0.76%
Total expenses     1.91%   2.08%   1.97%    1.60%   1.61%   1.87%
Net investment income 7.61% 8.23% 7.23% 3.57% 3.87% 3.81%
         
Senior Securities Amount $ 21,000 $ 12,000 $ 20,000 $ 26,000 $ 103,000 $ 95,000
Senior Securities Coverage per Unit [1] $ 8,085 $ 13,064 $ 8,235 $ 6,531 $ 3,903 $ 3,866
Preferred Stock Liquidating Preference [2] 25,000 25,000 25,000 25,000 25,000 25,000
Senior Securities Average Market Value per Unit [2] $ 25,000 25,000 25,000 25,000 25,000 25,000
General Description of Registrant [Abstract]            
Investment Objectives and Practices [Text Block] The Trust’s investment objective is to provide a high level of current income, consistent with the preservation of capital, by investing primarily in senior, secured floating-rate loans.          
Latest Premium (Discount) to NAV [Percent] [3] (0.16%)          
Common Shares [Member]            
Capital Stock, Long-Term Debt, and Other Securities [Abstract]            
Outstanding Security, Title [Text Block] Common Shares          
Outstanding Security, Held [Shares] 18,084,100          
Preferred Shares [Member]            
Financial Highlights [Abstract]            
Senior Securities Coverage per Unit [4] $ 72,434 $ 79,014 $ 71,481 $ 66,752 $ 71,484 $ 69,242
[1] Calculated by subtracting the Trust’s total liabilities (not including the notes payable and preferred shares) from the Trust’s total assets, and dividing the result by the notes payable balance in thousands.
[2] Plus accumulated and unpaid dividends.
[3] The shares of the Fund often trade at a discount or premium to their net asset value. The discount or premium may vary over time and may be higher or lower than what is quoted in this report. For up-to-date premium/discount information, please refer to https://funds.eatonvance.com/closed-end-fund-prices.php.
[4] Calculated by subtracting the Trust’s total liabilities (not including the notes payable and preferred shares) from the Trust’s total assets, dividing the result by the sum of the value of the notes payable and liquidation value of the preferred shares, and multiplying the result by the liquidation value of one preferred share.