| Shareholder Report | 6 Months Ended |
| Jun. 30, 2024 USD ($) Holding |
| Shareholder Report [Line Items] | |
| Document Type | N-CSRS |
| Amendment Flag | false |
| Registrant Name | BlackRock Series Fund II, Inc. |
| Entity Central Index Key | 0001738073 |
| Entity Investment Company Type | N-1A |
| Document Period End Date | Jun. 30, 2024 |
| C000202340 | |
| Shareholder Report [Line Items] | |
| Fund Name | BlackRock High Yield Portfolio |
| Trading Symbol | HIYLD |
| Annual or Semi-Annual Statement [Text Block] | This semi-annual shareholder report contains important information about BlackRock High Yield Portfolio (the “Fund”) for the period of January 1, 2024 to June 30, 2024. |
| Shareholder Report Annual or Semi-Annual | Semi-Annual Shareholder Report |
| Additional Information [Text Block] | You can find additional information about the Fund at blackrock.com/fundreports |
| Additional Information Phone Number | (800) 441-7762 |
| Additional Information Website | <span style="font-weight: 700; box-sizing: border-box; color: rgb(0, 0, 0); display: inline; flex-wrap: nowrap; font-size: 10.6667px; grid-area: auto; line-height: 16px; margin: 0px; overflow: visible; text-align: justify;">blackrock.com/fundreports</span></span>" id="sjs-B18"><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: justify;"><span style="font-weight: 700; box-sizing: border-box; color: rgb(0, 0, 0); display: inline; flex-wrap: nowrap; font-size: 10.6667px; grid-area: auto; line-height: 16px; margin: 0px; overflow: visible; text-align: justify;">blackrock.com/fundreports</span></span> |
| Expenses [Text Block] | What were the Fund costs for the last six months? (based on a hypothetical $10,000 investment) Fund name Costs of a $10,000 investment Costs paid as a percentage of a $10,000 investment BlackRock High Yield Portfolio $25 0.49% |
| Expenses Paid, Amount | $ 25 |
| Expense Ratio, Percent | 0.49% |
| Performance Past Does Not Indicate Future [Text] | <span style="font-weight: 700; box-sizing: border-box; color: rgb(0, 0, 0); display: inline; flex-wrap: nowrap; font-size: 10.6667px; grid-area: auto; line-height: 16px; margin: 0px; overflow: visible; text-align: justify;">Past performance is not an indication of future results. </span>Performance results may include adjustments made for financial reporting purposes in accordance with U.S. generally accepted accounting principles. Performance results do not reflect the deduction of taxes that a shareholder would pay on fund distributions or on the redemption or sale of fund shares. Visit <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: justify;"><span style="font-weight: 700; box-sizing: border-box; color: rgb(0, 0, 0); display: inline; flex-wrap: nowrap; font-size: 10.6667px; grid-area: auto; line-height: 16px; margin: 0px; overflow: visible; text-align: justify;">blackrock.com</span></span> for more recent performance information.</p>" id="sjs-B22"><p style="box-sizing: border-box; color: rgb(0, 0, 0); display: block; flex-wrap: nowrap; font-size: 10.6667px; font-weight: 400; grid-area: auto; line-height: 16px; margin: 0px; overflow: visible; text-align: justify;"><span style="font-weight: 700; box-sizing: border-box; color: rgb(0, 0, 0); display: inline; flex-wrap: nowrap; font-size: 10.6667px; grid-area: auto; line-height: 16px; margin: 0px; overflow: visible; text-align: justify;">Past performance is not an indication of future results. </span>Performance results may include adjustments made for financial reporting purposes in accordance with U.S. generally accepted accounting principles. Performance results do not reflect the deduction of taxes that a shareholder would pay on fund distributions or on the redemption or sale of fund shares. Visit <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: justify;"><span style="font-weight: 700; box-sizing: border-box; color: rgb(0, 0, 0); display: inline; flex-wrap: nowrap; font-size: 10.6667px; grid-area: auto; line-height: 16px; margin: 0px; overflow: visible; text-align: justify;">blackrock.com</span></span> for more recent performance information.</p> |
| Average Annual Return [Table Text Block] | Fund performance Average annual total returns Key Fund statistics Average Annual Total Returns 6-Month
Total Returns 1 Year 5 Years 10 Years
BlackRock High Yield Portfolio ................................................................................................................................................................ 3.04 % 10.43 % 4.40 % 4.26 % Bloomberg U.S. Universal Index ................................................................................................................................................................ (0.28 ) 3.47 0.11 1.63 Bloomberg U.S. Corporate High Yield 2% Issuer Capped Index ................................................................................................................................................................ 2.58 10.43 3.90 4.30 Net Assets ................................................................................................................................................................ $20,545,132 Number of Portfolio Holdings ................................................................................................................................................................ 707 Portfolio Turnover Rate ................................................................................................................................................................ 31% On September 17, 2018, the Fund acquired all of the assets, subject to the liabilities, of BlackRock High Yield Portfolio (the "Predecessor Fund"), a series of BlackRock Series Fund, Inc., through a tax-free reorganization (the "Reorganization"). The Predecessor Fund is the performance and accounting survivor of the Reorganization. Assuming transaction costs and other operating expenses, including investment advisory fees, if any. Average annual total returns do not reflect insurance-related fees and expenses. The Fund has added the Bloomberg U.S. Universal Index in response to new regulatory requirements. |
| No Deduction of Taxes [Text Block] | |
| Performance Table Market Index Changed [Text Block] | |
| Material Change Description [Text Block] | |
| Updated Performance Information Location [Text Block] | |
| AssetsNet | $ 20,545,132 |
| Holdings Count | Holding | 707 |
| InvestmentCompanyPortfolioTurnover | 31% |
| Holdings [Text Block] | Credit quality allocation Credit Rating Footnote Reference Percent of Total Investments Footnote Reference BBB/Baa ................................................................................................................................................................ 3.7 % BB/Ba ................................................................................................................................................................ 40.8 B ................................................................................................................................................................ 44.8 CCC/Caa ................................................................................................................................................................ 7.9 CC/Ca ................................................................................................................................................................ 0.0 Footnote Reference NR ................................................................................................................................................................ 2.8 Footnote Description Footnote (a) For purposes of this report, credit quality ratings shown above reflect the highest rating assigned by either S&P Global Ratings or Moody’s Investors Service, Inc. if ratings differ. These rating agencies are independent, nationally recognized statistical rating organizations and are widely used. Investment grade ratings are credit ratings of BBB/Baa or higher. Below investment grade ratings are credit ratings of BB/Ba or lower. Investments designated NR are not rated by either rating agency. Unrated investments do not necessarily indicate low credit quality. Credit quality ratings are subject to change. Footnote (b) Excludes short-term securities. Footnote (c) Rounds to less than 0.1%. |
