| Form N-CSR Cover | 12 Months Ended |
| Dec. 31, 2024 | |
| Shareholder Report [Line Items] | |
| Document Type | N-CSR |
| Amendment Flag | false |
| Registrant Name | William Blair Funds |
| Entity Central Index Key | 0000822632 |
| Entity Investment Company Type | N-1A |
| Document Period End Date | Dec. 31, 2024 |
| Shareholder Report | 12 Months Ended |
| Dec. 31, 2024 USD ($) Holding | |
| Shareholder Report [Line Items] | |
| Document Type | N-CSR |
| Amendment Flag | false |
| Registrant Name | William Blair Funds |
| Entity Central Index Key | 0000822632 |
| Entity Investment Company Type | N-1A |
| Document Period End Date | Dec. 31, 2024 |
| William Blair Growth Fund - Class I | |
| Shareholder Report [Line Items] | |
| Fund Name | William Blair Growth Fund |
| Class Name | Class I |
| Trading Symbol | BGFIX |
| Annual or Semi-Annual Statement [Text Block] | This annual shareholder report contains important information about William Blair Growth Fund Class I (the "Fund”) for the period of January 1, 2024, to December 31, 2024. |
| Shareholder Report Annual or Semi-Annual | annual shareholder report |
| Additional Information [Text Block] | You can find additional information about the Fund at https://www.williamblairfunds.com/literature/forms |
| Additional Information Phone Number | 1-800-742-7272 |
| Additional Information Website | https://www.williamblairfunds.com/literature/forms |
| Expenses [Text Block] | What were the Fund costs for the last year? (Based on a hypothetical $10,000 investment) Fund Cost of $10,000 Investment Cost of $10,000 Investment as a percentage Growth Fund Class I $ 0.89 % |
| Expenses Paid, Amount | $ 99 |
| Expense Ratio, Percent | 0.89% |
| Factors Affecting Performance [Text Block] | Management’s Discussion of Fund Performance SUMMARY OF RESULTS The Fund returned 22.21% (net of fees) for the 12 months ended December 31, 2024. The Fund underperformed its benchmark, the Russell 3000 ® ® The Fund underperformed in the full year period, driven by a combination of stock-specific dynamics and style factors. From a style perspective, our typical bias towards small and mid-caps was a headwind. Concurrently, mega caps markedly outperformed other capitalization segments in the period. From a sector perspective, Energy positions New Fortress and Green Plains served as detractors. Other individual laggards included Five Below (Consumer Discretionary), UnitedHealth Group (Health Care), and Advanced Micro Devices (Information Technology). Not owning Broadcom (Information Technology), Meta Platforms (Communication Services) and Tesla (Consumer Discretionary) also detracted from relative performance. Our top individual contributors included Vital Farms (Consumer Staples), Pure Storage (Information Technology), NVIDIA (Information Technology) and Doximity (Health Care). Our underweight to Apple (Information Technology) and not owning Adobe (Information Technology) also contributed positively to relative returns. TOP PERFORMANCE CONTRIBUTORS Vital Farms NVIDIA TOP PERFORMANCE DETRACTORS New Fortress Energy Five Below |
| Performance Past Does Not Indicate Future [Text] | Keep in mind that the Fund’s past performance is not a good predictor of how the Fund will perform in the future. |
| Line Graph [Table Text Block] | |
| Average Annual Return [Table Text Block] | Average Annual Total Returns 1 Year 5 Years 10 Years Growth Fund Class I 22.21 % 14.73 % 13.75 % S&P 500 ® 25.02 % 14.53 % 13.10 % Russell 3000 ® 32.46 % 18.25 % 16.22 % |
| No Deduction of Taxes [Text Block] | The graph and table do not reflect the deduction of taxes that a shareholder would pay on Fund distributions or redemption of Fund shares. |
| Updated Performance Information Location [Text Block] | Visit www.williamblairfunds.com/funds/total-returns |
| Net Assets | $ 249,916,000 |
| Holdings Count | Holding | 50 |
| Advisory Fees Paid, Amount | $ 1,891,000 |
| Investment Company, Portfolio Turnover | 49% |
| Additional Fund Statistics [Text Block] | Key Fund Statistic s Fund net assets (in $000s) $ Number of portfolio holdings 50 Net advisory fees paid (in $000s) $ Portfolio turnover rate 49 % |
| Holdings [Text Block] | Graphical Representation of Holdings The tables below show the in h Top Ten Holdings Amazon.com, Inc. 8.8 % Microsoft Corp. 8.6 % Alphabet, Inc. 7.2 % Apple, Inc. 4.9 % NVIDIA Corp. 4.8 % ServiceNow, Inc. 3.8 % Advanced Micro Devices, Inc. 3.5 % Mastercard, Inc. 3.5 % Texas Instruments, Inc. 2.8 % UnitedHealth Group, Inc. 2.6 % Sector Allocation |
| Largest Holdings [Text Block] | Top Ten Holdings Amazon.com, Inc. 8.8 % Microsoft Corp. 8.6 % Alphabet, Inc. 7.2 % Apple, Inc. 4.9 % NVIDIA Corp. 4.8 % ServiceNow, Inc. 3.8 % Advanced Micro Devices, Inc. 3.5 % Mastercard, Inc. 3.5 % Texas Instruments, Inc. 2.8 % UnitedHealth Group, Inc. 2.6 % |
| William Blair Growth Fund - Class N | |
| Shareholder Report [Line Items] | |
| Fund Name | William Blair Growth Fund |
| Class Name | Class N |
| Trading Symbol | WBGSX |
| Annual or Semi-Annual Statement [Text Block] | This annual shareholder report contains important information about William Blair Growth Fund Class N (the "Fund”) for the period of January 1, 2024, to December 31, 2024. |
| Shareholder Report Annual or Semi-Annual | annual shareholder report |
| Additional Information [Text Block] | You can find additional information about the Fund at https://www.williamblairfunds.com/literature/forms |
| Additional Information Phone Number | 1-800-742-7272 |
| Additional Information Website | https://www.williamblairfunds.com/literature/forms |
| Expenses [Text Block] | What were the Fund costs for the last year? (Based on a hypothetical $10,000 investment) Fund Cost of $10,000 Investment Cost of $10,000 Investment as a percentage Growth Fund Class N $ 1.20 % |
| Expenses Paid, Amount | $ 133 |
| Expense Ratio, Percent | 1.20% |
| Factors Affecting Performance [Text Block] | Management’s Discussion of Fund Perfo r SUMMARY OF RESULTS The Fund returned 21.81% (net of fees) for the 12 months ended December 31, 2024. The Fund underperformed its benchmark, the Russell 3000 ® ® The Fund underperformed in the full year period, driven by a combination of stock-specific dynamics and style factors. From a style perspective, our typical bias towards small and mid-caps was a headwind. Concurrently, mega caps markedly outperformed other capitalization segments in the period. From a sector perspective, Energy positions New Fortress and Green Plains served as detractors. Other individual laggards included Five Below (Consumer Discretionary), UnitedHealth Group (Health Care), and Advanced Micro Devices (Information Technology). Not owning Broadcom (Information Technology), Meta Platforms (Communication Services) and Tesla (Consumer Discretionary) also detracted from relative performance. Our top individual contributors included Vital Farms (Consumer Staples), Pure Storage (Information Technology), NVIDIA (Information Technology) and Doximity (Health Care). Our underweight to Apple (Information Technology) and not owning Adobe (Information Technology) also contributed positively to relative returns. TOP PERFORMANCE CONTRIBUTORS Vital Farms NVIDIA TOP PERFORMANCE DETRACTORS New Fortress Energy Five Below |
| Performance Past Does Not Indicate Future [Text] | Keep in mind that the Fund’s past performance is not a good predictor of how the Fund will perform in the future. |
| Line Graph [Table Text Block] | |
| Average Annual Return [Table Text Block] | Average Annual Total Returns 1 Year 5 Years 10 Years Growth Fund Class N 21.81 % 14.40 % 13.43 % S&P 500 ® 25.02 % 14.53 % 13.10 % Russell 3000 ® 32.46 % 18.25 % 16.22 % |
| No Deduction of Taxes [Text Block] | The graph and table do not reflect the deduction of taxes that a shareholder would pay on Fund distributions or redemption of Fund shares. |
| Updated Performance Information Location [Text Block] | Visit www.williamblairfunds.com/funds/total-returns |
| Net Assets | $ 249,916,000 |
| Holdings Count | Holding | 50 |
| Advisory Fees Paid, Amount | $ 1,891,000 |
| Investment Company, Portfolio Turnover | 49% |
| Additional Fund Statistics [Text Block] | Key Fund Statisti cs Fund net assets (in $000s) $ Number of portfolio holdings 50 Net advisory fees paid (in $000s) $ Portfolio turnover rate 49 % |
| Holdings [Text Block] | Graphical Representation of Holdings The tables below show the investment makeup of the Fund, representing percen t Top Ten Holdings Amazon.com, Inc. 8.8 % Microsoft Corp. 8.6 % Alphabet, Inc. 7.2 % Apple, Inc. 4.9 % NVIDIA Corp. 4.8 % ServiceNow, Inc. 3.8 % Advanced Micro Devices, Inc. 3.5 % Mastercard, Inc. 3.5 % Texas Instruments, Inc. 2.8 % UnitedHealth Group, Inc. 2.6 % Sector Allocation |
| Largest Holdings [Text Block] | Top Ten Holdings Amazon.com, Inc. 8.8 % Microsoft Corp. 8.6 % Alphabet, Inc. 7.2 % Apple, Inc. 4.9 % NVIDIA Corp. 4.8 % ServiceNow, Inc. 3.8 % Advanced Micro Devices, Inc. 3.5 % Mastercard, Inc. 3.5 % Texas Instruments, Inc. 2.8 % UnitedHealth Group, Inc. 2.6 % |
| William Blair Growth Fund - Class R6 | |
| Shareholder Report [Line Items] | |
| Fund Name | William Blair Growth Fund |
| Class Name | Class R6 |
| Trading Symbol | BGFRX |
| Annual or Semi-Annual Statement [Text Block] | This annual shareholder report contains important information about William Blair Growth Fund Class R6 (the "Fund”) for the period of January 1, 2024, to December 31, 2024. |
| Shareholder Report Annual or Semi-Annual | annual shareholder report |
| Additional Information [Text Block] | You can find additional information about the Fund at https://www.williamblairfunds.com/literature/forms |
| Additional Information Phone Number | 1-800-742-7272 |
| Additional Information Website | https://www.williamblairfunds.com/literature/forms |
| Expenses [Text Block] | What were the Fund costs for the last year? (Based on a hypothetical $10,000 investment) Fund Cost of $10,000 Investment Cost of $10,000 Investment as a percentage Growth Fund Class R6 $ 0.85 % |
| Expenses Paid, Amount | $ 94 |
| Expense Ratio, Percent | 0.85% |
| Factors Affecting Performance [Text Block] | Management’s Discussion of Fund Performance SUMMARY OF RESULTS The Fund returned 22.26% (net of fees) for the 12 months ended December 31, 2024. The Fund underperformed its benchmark, the Russell 3000 ® ® The Fund underperformed in the full year period, driven by a combination of stock-specific dynamics and style factors. From a style perspective, our typical bias towards small and mid-caps was a headwind. Concurrently, mega caps markedly outperformed other capitalization segments in the period. From a sector perspective, Energy positions New Fortress and Green Plains served as detractors. Other individual laggards included Five Below (Consumer Discretionary), UnitedHealth Group (Health Care), and Advanced Micro Devices (Information Technology). Not owning Broadcom (Information Technology), Meta Platforms (Communication Services) and Tesla (Consumer Discretionary) also detracted from relative performance. Our top individual contributors included Vital Farms (Consumer Staples), Pure Storage (Information Technology), NVIDIA (Information Technology) and Doximity (Health Care). Our underweight to Apple (Information Technology) and not owning Adobe (Information Technology) also contributed positively to relative returns. TOP PERFORMANCE CONTRIBUTORS Vital Farms NVIDIA TOP PERFORMANCE DETRACTORS New Fortress Energy Five Below |
| Performance Past Does Not Indicate Future [Text] | Keep in mind that the Fund’s past performance is not a good predictor of how the Fund will perform in the future. |
| Line Graph [Table Text Block] | |
| Average Annual Return [Table Text Block] | Average Annual Total Returns 1 Year 5 Years Since Inception Growth Fund Class R6 22.26 % 14.80 % 15.00 % S&P 500 ® 25.02 % 14.53 % 15.00 % Russell 3000 ® 32.46 % 18.25 % 18.44 % |
| No Deduction of Taxes [Text Block] | The graph and table do not reflect the deduction of taxes that a shareholder would pay on Fund distributions or redemption of Fund shares. |
| Updated Performance Information Location [Text Block] | Visit www.williamblairfunds.com/funds/total-returns |
| Net Assets | $ 249,916,000 |
| Holdings Count | Holding | 50 |
| Advisory Fees Paid, Amount | $ 1,891,000 |
| Investment Company, Portfolio Turnover | 49% |
| Additional Fund Statistics [Text Block] | Key Fund Statistic s Fund net assets (in $000s) $ Number of portfolio holdings 50 Net advisory fees paid (in $000s) $ Portfolio turnover rate 49 % |
| Holdings [Text Block] | Graphical Representation of Holdings The tables below show the investment makeup of the Fund, representing percentage of the total net assets of the Fund. Top Ten Holdings Amazon.com, Inc. 8.8 % Microsoft Corp. 8.6 % Alphabet, Inc. 7.2 % Apple, Inc. 4.9 % NVIDIA Corp. 4.8 % ServiceNow, Inc. 3.8 % Advanced Micro Devices, Inc. 3.5 % Mastercard, Inc. 3.5 % Texas Instruments, Inc. 2.8 % UnitedHealth Group, Inc. 2.6 % Sector Allocation |
| Largest Holdings [Text Block] | Top Ten Holdings Amazon.com, Inc. 8.8 % Microsoft Corp. 8.6 % Alphabet, Inc. 7.2 % Apple, Inc. 4.9 % NVIDIA Corp. 4.8 % ServiceNow, Inc. 3.8 % Advanced Micro Devices, Inc. 3.5 % Mastercard, Inc. 3.5 % Texas Instruments, Inc. 2.8 % UnitedHealth Group, Inc. 2.6 % |
| William Blair Large Cap Growth Fund - Class 1 | |
| Shareholder Report [Line Items] | |
| Fund Name | William Blair Large Cap Growth Fund |
| Class Name | Class I |
| Trading Symbol | LCGFX |
| Annual or Semi-Annual Statement [Text Block] | This annual shareholder report contains important information about William Blair Large Cap Growth Fund Class I (the "Fund”) for the period of January 1, 2024, to December 31, 2024. |
| Shareholder Report Annual or Semi-Annual | annual shareholder report |
| Additional Information [Text Block] | You can find additional information about the Fund at https://www.williamblairfunds.com/literature/forms |
| Additional Information Phone Number | 1-800-742-7272 |
| Additional Information Website | https://www.williamblairfunds.com/literature/forms |
| Expenses [Text Block] | What were the Fund costs for the last year? (Based on a hypothetical $10,000 investment) Fund Cost of $10,000 Investment Cost of $10,000 Investment as a percentage Large Cap Growth Fund Class I $ 0.65 % |
| Expenses Paid, Amount | $ 73 |
| Expense Ratio, Percent | 0.65% |
| Factors Affecting Performance [Text Block] | Management’s Discussion of Fund Performance SUMMARY OF RESULTS The Fund returned 26.08% (net of fees) for the 12 months ended December 31, 2024. The Fund underperformed its benchmark, the Russell 1000 ® ® The Fund lagged the return of the Russell 1000 Growth Index in the full year period, primarily driven by stock-specific dynamics. Communication Services, including our position in Alphabet and not owning Meta Platforms for a majority of the period, and Informa tio TOP PERFORMANCE CONTRIBUTORS NVIDIA Apollo Global Management TOP PERFORMANCE DETRACTORS Accenture c Monster Beverage |
| Performance Past Does Not Indicate Future [Text] | Keep in mind that the Fund’s past performance is not a good predictor of how the Fund will perform in the future. |
| Line Graph [Table Text Block] | |
| Average Annual Return [Table Text Block] | Average Annual Total Returns 1 Year 5 Years 10 Years Large Cap Growth Fund Class I 26.08 % 15.97 % 15.79 % Russell 1000® Index 24.51 % 14.28 % 12.87 % Russell 1000® Growth Index 33.36 % 18.96 % 16.78 % |
| No Deduction of Taxes [Text Block] | The graph and table do not reflect the deduction of taxes that a shareholder would pay on Fund distributions or redemption of Fund shares. |
| Updated Performance Information Location [Text Block] | Visit www.williamblairfunds.com/funds/total-returns |
| Net Assets | $ 2,803,223,000 |
| Holdings Count | Holding | 33 |
| Advisory Fees Paid, Amount | $ 12,317,000 |
| Investment Company, Portfolio Turnover | 50% |
| Additional Fund Statistics [Text Block] | Key Fund Statistics Fund net assets (in $000s) $ Number of portfolio holdings 33 Net advisory fees paid (in $000s) $ Portfolio turnover rate 50 % |
| Holdings [Text Block] | Graphical Representation of Holdings The tables below show the investment makeup of the Fund, representing percentage of the total net assets of the Fund. Top Ten Holdings Microsoft Corp. 12.2 % NVIDIA Corp. 11.9 % Amazon.com, Inc. 9.5 % Apple, Inc. 9.0 % Meta Platforms, Inc. 6.5 % Mastercard, Inc. 4.7 % Salesforce, Inc. 2.8 % ServiceNow, Inc. 2.5 % UnitedHealth Group, Inc. 2.4 % Intuit, Inc. 2.3 % Sector Allocation |
| Largest Holdings [Text Block] | Top Ten Holdings Microsoft Corp. 12.2 % NVIDIA Corp. 11.9 % Amazon.com, Inc. 9.5 % Apple, Inc. 9.0 % Meta Platforms, Inc. 6.5 % Mastercard, Inc. 4.7 % Salesforce, Inc. 2.8 % ServiceNow, Inc. 2.5 % UnitedHealth Group, Inc. 2.4 % Intuit, Inc. 2.3 % |
| William Blair Large Cap Growth Fund - Class N | |
| Shareholder Report [Line Items] | |
| Fund Name | William Blair Large Cap Growth Fund |
| Class Name | Class N |
| Trading Symbol | LCGNX |
| Annual or Semi-Annual Statement [Text Block] | This annual shareholder report contains important information about William Blair Large Cap Growth Fund Class N (the "Fund”) for the period of January 1, 2024, to December 31, 2024. |
| Shareholder Report Annual or Semi-Annual | annual shareholder report |
| Additional Information [Text Block] | You can find additional information about the Fund at https://www.williamblairfunds.com/literature/forms |
| Additional Information Phone Number | 1-800-742-7272 |
| Additional Information Website | https://www.williamblairfunds.com/literature/forms |
| Expenses [Text Block] | What were the Fund costs for the last year? (Based on a hypothetical $10,000 investment) Fund Cost of $10,000 Investment Cost of $10,000 Investment as a percentage Large Cap Growth Fund Class N $ 0.90 % |
| Expenses Paid, Amount | $ 102 |
| Expense Ratio, Percent | 0.90% |
| Factors Affecting Performance [Text Block] | Management’s Discussion of Fund Performance SUMMARY OF RESULTS The Fund returned 25.79% (net of fees) for the 12 months ended December 31, 2024. The Fund underperformed its benchmark, the Russell 1000 ® ® The Fund lagged the return of the Russell 1000 Growth Index in the full year period, primarily driven by stock-specific dynamics. Communication Services, including our position in Alphabet and not owning Meta Platforms for a majority of the period, and Information Technology, including our position in Accenture and not owning Broadcom for a majority of the period detracted from returns. Other detractors included Monster Beverage Corp. (Consumer Staples). Not owning Tesla (Consumer Discretionary) was also a headwind. The top contributors for the period included NVIDIA (Inf o TOP PERFORMANCE CONTRIBUTORS NVIDIA Apollo Global Management TOP PERFORMANCE DETRACTORS Accenture Monster Beverage |
| Performance Past Does Not Indicate Future [Text] | Keep in mind that the Fund’s past performance is not a good predictor of how the Fund will perform in the future. |
| Line Graph [Table Text Block] | |
| Average Annual Return [Table Text Block] | Average Annual Total Returns 1 Year 5 Years 10 Years Large Cap Growth Fund Class N 25.79 % 15.69 % 15.50 % Russell 1000® Index 24.51 % 14.28 % 12.87 % Russell 1000® Growth Index 33.36 % 18.96 % 16.78 % |
| No Deduction of Taxes [Text Block] | The graph and table do not reflect the deduction of taxes that a shareholder would pay on Fund distributions or redemption of Fund shares. |
| Updated Performance Information Location [Text Block] | Visit www.williamblairfunds.com/funds/total-returns |
| Net Assets | $ 2,803,223,000 |
| Holdings Count | Holding | 33 |
| Advisory Fees Paid, Amount | $ 12,317,000 |
| Investment Company, Portfolio Turnover | 50% |
| Additional Fund Statistics [Text Block] | Key Fund Statistics Fund net assets (in $000s) $ Number of portfolio holdings 33 Net advisory fees paid (in $000s) $ Portfolio turnover rate 50 % |
| Holdings [Text Block] | Graphical Representation of Holdings The tables below show the investment makeup of the Fund, representing percentage of the total net assets of the Fund. Top Ten Holdings Microsoft Corp. 12.2 % NVIDIA Corp. 11.9 % Amazon.com, Inc. 9.5 % Apple, Inc. 9.0 % Meta Platforms, Inc. 6.5 % Mastercard, Inc. 4.7 % Salesforce, Inc. 2.8 % ServiceNow, Inc. 2.5 % UnitedHealth Group, Inc. 2.4 % Intuit, Inc. 2.3 % Sector Allocation |
| Largest Holdings [Text Block] | Top Ten Holdings Microsoft Corp. 12.2 % NVIDIA Corp. 11.9 % Amazon.com, Inc. 9.5 % Apple, Inc. 9.0 % Meta Platforms, Inc. 6.5 % Mastercard, Inc. 4.7 % Salesforce, Inc. 2.8 % ServiceNow, Inc. 2.5 % UnitedHealth Group, Inc. 2.4 % Intuit, Inc. 2.3 % |
| William Blair Large Cap Growth Fund - Class R6 | |
| Shareholder Report [Line Items] | |
| Fund Name | William Blair Large Cap Growth Fund |
| Class Name | Class R6 |
| Trading Symbol | LCGJX |
| Annual or Semi-Annual Statement [Text Block] | This annual shareholder report contains important information about William Blair Large Cap Growth Fund Class R6 (the "Fund”) for the period of January 1, 2024, to December 31, 2024. |
| Shareholder Report Annual or Semi-Annual | annual shareholder report |
| Additional Information [Text Block] | You can find additional information about the Fund at https://www.williamblairfunds.com/literature/forms |
| Additional Information Phone Number | 1-800-742-7272 |
| Additional Information Website | https://www.williamblairfunds.com/literature/forms |
| Expenses [Text Block] | What were the Fund costs for the last year? (Based on a hypothetical $10,000 investment) Fund Cost of $10,000 Investment Cost of $10,000 Investment as a percentage Large Cap Growth Fund Class R6 $ 0.60 % |
| Expenses Paid, Amount | $ 68 |
| Expense Ratio, Percent | 0.60% |
| Factors Affecting Performance [Text Block] | Management’s Discussion of Fund Performance SUMMARY OF RESULTS The Fund returned 26.14% (net of fees) for the 12 months ended December 31, 2024. The Fund underperformed its benchmark, the Russell 1000 ® ® The Fund lagged the return of the Russell 1000 Growth Index in the full year period, primarily driven by stock-specific dynamics. Communication Services, including our position in Alphabet and not owning Meta Platforms for a majority of the period, and Information Technology, including our position in Accenture and not owning Broadcom for a majority of the period detracted from returns. Other detractors included Monster Beverage Corp. (Consumer Staples). Not owning Tesla (Consumer Discretionary) was also a headwind. The top contributors for the period included NVIDIA (Information Technology), Apollo Global Management (Financials), ServiceNow (Information Technology), Intuitive Surgical (Health Care), as well as our underweight to Apple (Information Technology). Not owning Adobe (Information Technology) and Merck & Co. (Health Care) were also tailwinds to performance for the period. TOP PERFORMANCE CONTRIBUTORS NVIDIA Apollo Global Management TOP PERFORMANCE DETRACTORS Accenture Monster Beverage |
| Performance Past Does Not Indicate Future [Text] | Keep in mind that the Fund’s past performance is not a good predictor of how the Fund will perform in the future. |
| Line Graph [Table Text Block] | |
| Average Annual Return [Table Text Block] | Average Annual Total Returns 1 Year 5 Years Since Inception Large Cap Growth Fund Class R6 26.14 % 16.04 % 16.71 % Russell 1000® Index 24.51 % 14.28 % 14.69 % Russell 1000® Growth Index 33.36 % 18.96 % 19.14 % |
| No Deduction of Taxes [Text Block] | The graph and table do not reflect the deduction of taxes that a shareholder would pay on Fund distributions or redemption of Fund shares. |
| Updated Performance Information Location [Text Block] | Visit www.williamblairfunds.com/funds/total-returns |
| Net Assets | $ 2,803,223,000 |
| Holdings Count | Holding | 33 |
| Advisory Fees Paid, Amount | $ 12,317,000 |
| Investment Company, Portfolio Turnover | 50% |
| Additional Fund Statistics [Text Block] | Key Fund Statistics Fund net assets (in $000s) $ Number of portfolio holdings 33 Net advisory fees paid (in $000s) $ Portfolio turnover rate 50 % |
| Holdings [Text Block] | Graphical Representation of Holdings The tables below show the investment makeup of the Fund, representing percentage of the total net assets of the Fund. Top Ten Holdings Microsoft Corp. 12.2 % NVIDIA Corp. 11.9 % Amazon.com, Inc. 9.5 % Apple, Inc. 9.0 % Meta Platforms, Inc. 6.5 % Mastercard, Inc. 4.7 % Salesforce, Inc. 2.8 % ServiceNow, Inc. 2.5 % UnitedHealth Group, Inc. 2.4 % Intuit, Inc. 2.3 % Sector Allocation |
| Largest Holdings [Text Block] | Top Ten Holdings Microsoft Corp. 12.2 % NVIDIA Corp. 11.9 % Amazon.com, Inc. 9.5 % Apple, Inc. 9.0 % Meta Platforms, Inc. 6.5 % Mastercard, Inc. 4.7 % Salesforce, Inc. 2.8 % ServiceNow, Inc. 2.5 % UnitedHealth Group, Inc. 2.4 % Intuit, Inc. 2.3 % |
| William Blair Mid Cap Value Fund - Class I | |
| Shareholder Report [Line Items] | |
| Fund Name | William Blair Mid Cap Value Fund |
| Class Name | Class I |
| Trading Symbol | WVMIX |
| Annual or Semi-Annual Statement [Text Block] | This annual shareholder report contains important information about William Blair Mid Cap Value Fund Class I (the "Fund”) for the period of January 1, 2024, to December 31, 2024. |
| Shareholder Report Annual or Semi-Annual | annual shareholder report |
| Additional Information [Text Block] | You can find additional information about the Fund at https://www.williamblairfunds.com/literature/forms |
| Additional Information Phone Number | 1-800-742-7272 |
| Additional Information Website | https://www.williamblairfunds.com/literature/forms |
| Expenses [Text Block] | What were the Fund costs for the last year? (Based on a hypothetical $10,000 investment) Fund Cost of $10,000 Investment Cost of $10,000 Investment as a percentage Mid Cap Value Fund Class I $ 0.75 % |
| Expenses Paid, Amount | $ 79 |
| Expense Ratio, Percent | 0.75% |
| Factors Affecting Performance [Text Block] | Management’s Discussion of Fund Performance SUMMARY OF RESULTS The Fund returned 10.71% (net of fees) for the 12 months ended December 31, 2024. The Fund underperformed its benchmark, the Russell Midcap ® ® The Fund’s underperformance versus its benchmark was driven by poor sector allocation due to the Fund’s underweight in the Health Care and Financials sectors. Despite negative performance, the Fund had positive stock selection in 5 of the 11 Global Industry Classification Standard economic sectors during the period. The Fund’s positive stock selections were additive to relative performance in the Consumer Staples, Utilities, and Health Care sectors. TOP PERFORMANCE CONTRIBUTORS KKR & Co. Alaska Air Group Flex TOP PERFORMANCE DETRACTORS Brunswick Qorvo LKQ |
| Performance Past Does Not Indicate Future [Text] | Keep in mind that the Fund’s past performance is not a good predictor of how the Fund will perform in the future. |
| Line Graph [Table Text Block] | |
| Average Annual Return [Table Text Block] | Average Annual Total Returns 1 Year Since Inception Mid Cap Value Fund Class I 10.71 % 5.57 % Russell 3000 ® 23.81 % 13.25 % Russell Midcap ® 13.07 % 6.65 % |
| No Deduction of Taxes [Text Block] | The graph and table do not reflect the deduction of taxes that a shareholder would pay on Fund distributions or redemption of Fund shares. |
| Updated Performance Information Location [Text Block] | Visit www.williamblairfunds.com/funds/total-returns |
| Net Assets | $ 1,757,000 |
| Holdings Count | Holding | 61 |
| Advisory Fees Paid, Amount | $ 0 |
| Investment Company, Portfolio Turnover | 39% |
| Additional Fund Statistics [Text Block] | Key Fund Statistics Fund net assets (in $000s) $ Number of portfolio holdings 61 Net advisory fees paid (in $000s) $ Portfolio turnover rate 39 % |
| Holdings [Text Block] | Graphical Representation of Holdings The tables below show the investment makeup of the Fund, representing percentage of the total net assets of the Fund. Top Ten Holdings Willis Towers Watson PLC 2.1 % CRH PLC 2.0 % Regency Centers Corp. 2.0 % Genpact Ltd. 1.9 % PPL Corp. 1.9 % U.S. Foods Holding Corp. 1.9 % KKR & Co., Inc. 1.9 % Entergy Corp. 1.8 % Global Payments, Inc. 1.8 % NiSource, Inc. 1.8 % Sector Allocation |
| Largest Holdings [Text Block] | Top Ten Holdings Willis Towers Watson PLC 2.1 % CRH PLC 2.0 % Regency Centers Corp. 2.0 % Genpact Ltd. 1.9 % PPL Corp. 1.9 % U.S. Foods Holding Corp. 1.9 % KKR & Co., Inc. 1.9 % Entergy Corp. 1.8 % Global Payments, Inc. 1.8 % NiSource, Inc. 1.8 % |
| William Blair Mid Cap Value Fund - Class R6 | |
| Shareholder Report [Line Items] | |
| Fund Name | William Blair Mid Cap Value Fund |
| Class Name | Class R6 |
| Trading Symbol | WVMRX |
| Annual or Semi-Annual Statement [Text Block] | This annual shareholder report contains important information about William Blair Mid Cap Value Fund Class R6 (the "Fund”) for the period of January 1, 2024, to December 31, 2024. |
| Shareholder Report Annual or Semi-Annual | annual shareholder report |
| Additional Information [Text Block] | You can find additional information about the Fund at https://www.williamblairfunds.com/literature/forms |
| Additional Information Phone Number | 1-800-742-7272 |
| Additional Information Website | https://www.williamblairfunds.com/literature/forms |
| Expenses [Text Block] | What were the Fund costs for the last year? (Based on a hypothetical $10,000 investment) Fund Cost of $10,000 Investment Cost of $10,000 Investment as a percentage Mid Cap Value Fund Class R6 $ 0.70 % |
| Expenses Paid, Amount | $ 74 |
| Expense Ratio, Percent | 0.70% |
| Factors Affecting Performance [Text Block] | Management’s Discussion of Fund Performance SUMMARY OF RESULTS The Fund returned 10.77% (net of fees) for the 12 months ended December 31, 2024. The Fund underperformed its benchmark, the Russell Midcap ® ® The Fund’s underperformance versus its benchmark was driven by poor sector allocation due to the Fund’s underweight in the Health Care and Financials sectors. Despite negative performance, the Fund had positive stock selection in 5 of the 11 Global Industry Classification Standard economic sectors during the period. The Fund’s positive stock selections were additive to relative performance in the Consumer Staples, Utilities, and Health Care sectors. TOP PERFORMANCE CONTRIBUTORS KKR & Co. Alaska Air Group Flex TOP PERFORMANCE DETRACTORS Brunswick Qorvo LKQ |
| Performance Past Does Not Indicate Future [Text] | Keep in mind that the Fund’s past performance is not a good predictor of how the Fund will perform in the future. |
| Line Graph [Table Text Block] | |
| Average Annual Return [Table Text Block] | Average Annual Total Returns 1 Year Since Inception Mid Cap Value Fund Class R6 10.77 % 5.62 % Russell 3000 ® 23.81 % 13.25 % Russell Midcap ® 13.07 % 6.65 % |
| No Deduction of Taxes [Text Block] | The graph and table do not reflect the deduction of taxes that a shareholder would pay on Fund distributions or redemption of Fund shares. |
| Updated Performance Information Location [Text Block] | Visit www.williamblairfunds.com/funds/total-returns |
| Net Assets | $ 1,757,000 |
| Holdings Count | Holding | 61 |
| Advisory Fees Paid, Amount | $ 0 |
| Investment Company, Portfolio Turnover | 39% |
| Additional Fund Statistics [Text Block] | Key Fund Statistics Fund net assets (in $000s) $ Number of portfolio holdings 61 Net advisory fees paid (in $000s) $ Portfolio turnover rate 39 % |
| Holdings [Text Block] | Graphical Representation of Holdings The tables below show the investment makeup of the Fund, representing percentage of the total net assets of the Fund. Top Ten Holdings Willis Towers Watson PLC 2.1 % CRH PLC 2.0 % Regency Centers Corp. 2.0 % Genpact Ltd. 1.9 % PPL Corp. 1.9 % U.S. Foods Holding Corp. 1.9 % KKR & Co., Inc. 1.9 % Entergy Corp. 1.8 % Global Payments, Inc. 1.8 % NiSource, Inc. 1.8 % Sector Allocation |
| Largest Holdings [Text Block] | Top Ten Holdings Willis Towers Watson PLC 2.1 % CRH PLC 2.0 % Regency Centers Corp. 2.0 % Genpact Ltd. 1.9 % PPL Corp. 1.9 % U.S. Foods Holding Corp. 1.9 % KKR & Co., Inc. 1.9 % Entergy Corp. 1.8 % Global Payments, Inc. 1.8 % NiSource, Inc. 1.8 % |
| William Blair Small-Mid Cap Core Fund - Class I | |
| Shareholder Report [Line Items] | |
| Fund Name | William Blair Small-Mid CapCore Fund |
| Class Name | Class I |
| Trading Symbol | WBCIX |
| Annual or Semi-Annual Statement [Text Block] | This annual shareholder report contains important information about William Blair Small-Mid Cap Core Fund Class I (the "Fund”) for the period of January 1, 2024, to December 31, 2024. |
| Shareholder Report Annual or Semi-Annual | annual shareholder report |
| Additional Information [Text Block] | You can find additional information about the Fund at https://www.williamblairfunds.com/literature/forms |
| Additional Information Phone Number | 1-800-742-7272 |
| Additional Information Website | https://www.williamblairfunds.com/literature/forms |
| Expenses [Text Block] | What were the Fund costs for the last year? (Based on a hypothetical $10,000 investment) Fund Cost of $10,000 Investment Cost of $10,000 Investment as a percentage Small-Mid Cap Core Fund Class I $ 0.95 % |
| Expenses Paid, Amount | $ 101 |
| Expense Ratio, Percent | 0.95% |
| Factors Affecting Performance [Text Block] | Management’s Discussion of Fund Performance SUMMARY OF RESULTS The Fund returned 12.04% (net of fees) for the 12 months ended December 31, 2024. The Fund outperformed its benchmark, the Russell 2500 ™ ® The Fund performed approximately in line with the performance of the Russell 2500 TM TOP PERFORMANCE CONTRIBUTORS Baldwin Insurance BWX Technologies TOP PERFORMANCE DETRACTORS New Fortress Energy Acadia Healthcare |
| Performance Past Does Not Indicate Future [Text] | Keep in mind that the Fund’s past performance is not a good predictor of how the Fund will perform in the future. |
| Line Graph [Table Text Block] | |
| Average Annual Return [Table Text Block] | Average Annual Total Returns 1 Year 5 Years Since Inception Small-Mid Cap Core Fund Class I 12.04 % 9.94 % 10.84 % Russell 3000® Index 23.81 % 13.86 % 15.03 % Russell 2500™ Index 12.00 % 8.77 % 10.04 % |
| No Deduction of Taxes [Text Block] | The graph and table do not reflect the deduction of taxes that a shareholder would pay on Fund distributions or redemption of Fund shares. |
| Updated Performance Information Location [Text Block] | Visit www.williamblairfunds.com/funds/total-returns |
| Net Assets | $ 124,651,000 |
| Holdings Count | Holding | 89 |
| Advisory Fees Paid, Amount | $ 1,444,000 |
| Investment Company, Portfolio Turnover | 59% |
| Additional Fund Statistics [Text Block] | Key Fund Statistics Fund net assets (in $000s) $ Number of portfolio holdings 89 Net advisory fees paid (in $000s) $ Portfolio turnover rate 59 % |
| Holdings [Text Block] | Graphical Representation of Holdings The tables below show the investment makeup percentage Top Ten Holdings Carlyle Group, Inc. 2.5 % SharkNinja, Inc. 2.2 % Western Alliance Bancorp 2.1 % Primo Brands Corporation 1.9 % Bright Horizons Family Solutions, Inc. 1.9 % TransUnion 1.8 % Everest Group Ltd. 1.8 % ACV Auctions, Inc. 1.8 % Old National Bancorp 1.7 % Chemed Corp. 1.7 % Sector Allocation |
| Largest Holdings [Text Block] | Top Ten Holdings Carlyle Group, Inc. 2.5 % SharkNinja, Inc. 2.2 % Western Alliance Bancorp 2.1 % Primo Brands Corporation 1.9 % Bright Horizons Family Solutions, Inc. 1.9 % TransUnion 1.8 % Everest Group Ltd. 1.8 % ACV Auctions, Inc. 1.8 % Old National Bancorp 1.7 % Chemed Corp. 1.7 % |
| William Blair Small-Mid Cap Core Fund - Class R6 | |
| Shareholder Report [Line Items] | |
| Fund Name | William Blair Small-Mid CapCore Fund |
| Class Name | Class R6 |
| Trading Symbol | WBCRX |
| Annual or Semi-Annual Statement [Text Block] | This annual shareholder report contains important information about William Blair Small-Mid Cap Core Fund Class R6 (the "Fund”) for the period of January 1, 2024, to December 31, 2024. |
| Shareholder Report Annual or Semi-Annual | annual shareholder report |
| Additional Information [Text Block] | You can find additional information about the Fund at https://www.williamblairfunds.com/literature/forms |
| Additional Information Phone Number | 1-800-742-7272 |
| Additional Information Website | https://www.williamblairfunds.com/literature/forms |
| Expenses [Text Block] | What were the Fund costs for the last year? (Based on a hypothetical $10,000 investment) Fund Cost of $10,000 Investment Cost of $10,000 Investment as a percentage Small-Mid Cap Core Fund Class R6 $ 0.90 % |
| Expenses Paid, Amount | $ 95 |
| Expense Ratio, Percent | 0.90% |
| Factors Affecting Performance [Text Block] | Management’s Discussion of Fund Performance SUMMARY OF RESULTS The Fund returned 12.16% (net of fees) for the 12 months ended December 31, 2024. The Fund outperformed its benchmark, the Russell 2500 ™ ® The Fund performed approximately in line with the performance of the Russell 2500 TM TOP PERFORMANCE CONTRIBUTORS Baldwin Insurance BWX Technologies TOP PERFORMANCE DETRACTORS New Fortress Energy Acadia Healthcare |
| Performance Past Does Not Indicate Future [Text] | Keep in mind that the Fund’s past performance is not a good predictor of how the Fund will perform in the future. |
| Line Graph [Table Text Block] | |
| Average Annual Return [Table Text Block] | Average Annual Total Returns 1 Year 5 Years Since Inception Small-Mid Cap Core Fund Class R6 12.16 % 9.99 % 10.88 % Russell 3000® Index 23.81 % 13.86 % 15.03 % Russell 2500™ Index 12.00 % 8.77 % 10.04 % |
| No Deduction of Taxes [Text Block] | The graph and table do not reflect the deduction of taxes that a shareholder would pay on Fund distributions or redemption of Fund shares. |
| Updated Performance Information Location [Text Block] | Visit www.williamblairfunds.com/funds/total-returns |
| Net Assets | $ 124,651,000 |
| Holdings Count | Holding | 89 |
| Advisory Fees Paid, Amount | $ 1,444,000 |
| Investment Company, Portfolio Turnover | 59% |
| Additional Fund Statistics [Text Block] | Key Fund Statistics Fund net assets (in $000s) $ Number of portfolio holdings 89 Net advisory fees paid (in $000s) $ Portfolio turnover rate 59 % |
| Holdings [Text Block] | Graphical Representation of Holdings The tables below show the investment Top Ten Holdings Carlyle Group, Inc. 2.5 % SharkNinja, Inc. 2.2 % Western Alliance Bancorp 2.1 % Primo Brands Corporation 1.9 % Bright Horizons Family Solutions, Inc. 1.9 % TransUnion 1.8 % Everest Group Ltd. 1.8 % ACV Auctions, Inc. 1.8 % Old National Bancorp 1.7 % Chemed Corp. 1.7 % Sector Allocation |
| Largest Holdings [Text Block] | Top Ten Holdings Carlyle Group, Inc. 2.5 % SharkNinja, Inc. 2.2 % Western Alliance Bancorp 2.1 % Primo Brands Corporation 1.9 % Bright Horizons Family Solutions, Inc. 1.9 % TransUnion 1.8 % Everest Group Ltd. 1.8 % ACV Auctions, Inc. 1.8 % Old National Bancorp 1.7 % Chemed Corp. 1.7 % |
| William Blair Small-Mid Cap Growth Fund - Class I | |
| Shareholder Report [Line Items] | |
| Fund Name | William Blair Small-Mid CapGrowth Fund |
| Class Name | Class I |
| Trading Symbol | WSMDX |
| Annual or Semi-Annual Statement [Text Block] | This annual shareholder report contains important information about William Blair Small-Mid Cap Growth Fund Class I (the "Fund”) for the period of January 1, 2024, to December 31, 2024. |
| Shareholder Report Annual or Semi-Annual | annual shareholder report |
| Additional Information [Text Block] | You can find additional information about the Fund at https://www.williamblairfunds.com/literature/forms |
| Additional Information Phone Number | 1-800-742-7272 |
| Additional Information Website | https://www.williamblairfunds.com/literature/forms |
| Expenses [Text Block] | What were the Fund costs for the last year? (Based on a hypothetical $10,000 investment) Fund Cost of $10,000 Investment Cost of $10,000 Investment as a percentage Small-Mid Cap Growth Fund Class I $ 0.99 % |
| Expenses Paid, Amount | $ 105 |
| Expense Ratio, Percent | 0.99% |
| Factors Affecting Performance [Text Block] | Management’s Discussion of Fund Performance SUMMARY OF RESULTS The Fund returned 11.51% (net of fees) for the 12 months ended December 31, 2024. The Fund underperformed its benchmark, the Russell 2500 ™ ® The Fund lagged the return of the Russell 2500 Growth Index in the full year period, primarily driven by stock-specific dynamics. Stock selection in Energy, including our position in New Fortress, detracted from performance. Other notable detractors included Acadia Healthcare (Health Care), Evolent Health (Health Care), Neogen Corp (Health Care) and National Vision (Consumer Discretionary). Positive stock selection in Consumer Staples, including our positions in Freshpet and Primo Brands, added value during the period. Other top contributors included Doximity (Health Care), BWX Technologies (Industrials) and Insmed (Health Care). TOP PERFORMANCE CONTRIBUTORS Freshpet Doximity TOP PERFORMANCE DETRACTORS New Fortress Energy Acadia Healthcare |
| Performance Past Does Not Indicate Future [Text] | Keep in mind that the Fund’s past performance is not a good predictor of how the Fund will perform in the future. |
| Line Graph [Table Text Block] | |
| Average Annual Return [Table Text Block] | Average Annual Total Returns 1 Year 5 Years 10 Years Small-Mid Cap Growth Fund Class I 11.51 % 7.81 % 10.39 % Russell 3000 ® 23.81 % 13.86 % 12.55 % Russell 2500 ™ 13.90 % 8.08 % 9.45 % |
| No Deduction of Taxes [Text Block] | The graph and table do not reflect the deduction of taxes that a shareholder would pay on Fund distributions or redemption of Fund shares. |
| Updated Performance Information Location [Text Block] | Visit www.williamblairfunds.com/funds/total-returns |
| Net Assets | $ 1,887,232,000 |
| Holdings Count | Holding | 80 |
| Advisory Fees Paid, Amount | $ 16,315,000 |
| Investment Company, Portfolio Turnover | 63% |
| Additional Fund Statistics [Text Block] | Key Fund Statistics Fund net assets (in $000s) $ Number of portfolio holdings 80 Net advisory fees paid (in $000s) $ Portfolio turnover rate 63 % |
| Holdings [Text Block] | Graphical Representation of Holdings The tables below show the investment makeup of the Fund, representing percentage of the total net assets of the Fund. Top Ten Holdings Carlyle Group, Inc. 2.3 % Primo Brands Corporation 2.3 % e.l.f. Beauty, Inc. 2.1 % Doximity, Inc. 2.1 % Dynatrace, Inc. 2.0 % Talen Energy Corporation 2.0 % Confluent, Inc. 2.0 % SharkNinja, Inc. 2.0 % Bright Horizons Family Solutions, Inc. 1.8 % Chemed Corp. 1.8 % Sector Allocation |
| Largest Holdings [Text Block] | Top Ten Holdings Carlyle Group, Inc. 2.3 % Primo Brands Corporation 2.3 % e.l.f. Beauty, Inc. 2.1 % Doximity, Inc. 2.1 % Dynatrace, Inc. 2.0 % Talen Energy Corporation 2.0 % Confluent, Inc. 2.0 % SharkNinja, Inc. 2.0 % Bright Horizons Family Solutions, Inc. 1.8 % Chemed Corp. 1.8 % |
| William Blair Small-Mid Cap Growth Fund - Class N | |
| Shareholder Report [Line Items] | |
| Fund Name | William Blair Small-Mid CapGrowth Fund |
| Class Name | Class N |
| Trading Symbol | WSMNX |
| Annual or Semi-Annual Statement [Text Block] | This annual shareholder report contains important information about William Blair Small-Mid Cap Growth Fund Class N (the "Fund”) for the period of January 1, 2024, to December 31, 2024. |
| Shareholder Report Annual or Semi-Annual | annual shareholder report |
| Additional Information [Text Block] | You can find additional information about the Fund at https://www.williamblairfunds.com/literature/forms |
| Additional Information Phone Number | 1-800-742-7272 |
| Additional Information Website | https://www.williamblairfunds.com/literature/forms |
| Expenses [Text Block] | What were the Fund costs for the last year? (Based on a hypothetical $10,000 investment) Fund Cost of $10,000 Investment Cost of $10,000 Investment as a percentage Small-Mid Cap Growth Fund Class N $ 1.24 % |
| Expenses Paid, Amount | $ 131 |
| Expense Ratio, Percent | 1.24% |
| Factors Affecting Performance [Text Block] | Management’s Discussion of Fund Performance SUMMARY OF RESULTS The Fund returned 11.21% (net of fees) for the 12 months ended December 31, 2024. The Fund underperformed its benchmark, the Russell 2500 ™ ® The Fund lagged the return of the Russell 2500 Growth Index in the full year period, primarily driven by stock-specific dynamics. Stock selection in Energy, including our position in New Fortress, detracted from performance. Other notable detractors included Acadia Healthcare (Health Care), Evolent Health (Health Care), Neogen Corp (Health Care) and National Vision (Consumer Discretionary). Positive stock selection in Consumer Staples, including our positions in Freshpet and Primo Brands, added value during the period. Other top contributors included Doximity (Health Care), BWX Technologies (Industrials) and Insmed (Health Care). TOP PERFORMANCE CONTRIBUTORS Freshpet Doximity TOP PERFORMANCE DETRACTORS New Fortress Energy Acadia Healthcare |
| Performance Past Does Not Indicate Future [Text] | Keep in mind that the Fund’s past performance is not a good predictor of how the Fund will perform in the future. |
| Line Graph [Table Text Block] | |
| Average Annual Return [Table Text Block] | Average Annual Total Returns 1 Year 5 Years 10 Years Small-Mid Cap Growth Fund Class N 11.21 % 7.54 % 10.11 % Russell 3000 ® 23.81 % 13.86 % 12.55 % Russell 2500 ™ 13.90 % 8.08 % 9.45 % |
| No Deduction of Taxes [Text Block] | The graph and table do not reflect the deduction of taxes that a shareholder would pay on Fund distributions or redemption of Fund shares. |
| Updated Performance Information Location [Text Block] | Visit www.williamblairfunds.com/funds/total-returns |
| Net Assets | $ 1,887,232,000 |
| Holdings Count | Holding | 80 |
| Advisory Fees Paid, Amount | $ 16,315,000 |
| Investment Company, Portfolio Turnover | 63% |
| Additional Fund Statistics [Text Block] | Key Fund Statistics Fund net assets (in $000s) $ Number of portfolio holdings 80 Net advisory fees paid (in $000s) $ Portfolio turnover rate 63 % |
| Holdings [Text Block] | Graphical Representation of Holdings The tables below show the investment makeup of the Fund, representing percentage of the total net assets of the Fund. Top Ten Holdings Carlyle Group, Inc. 2.3 % Primo Brands Corporation 2.3 % e.l.f. Beauty, Inc. 2.1 % Doximity, Inc. 2.1 % Dynatrace, Inc. 2.0 % Talen Energy Corporation 2.0 % Confluent, Inc. 2.0 % SharkNinja, Inc. 2.0 % Bright Horizons Family Solutions, Inc. 1.8 % Chemed Corp. 1.8 % Sector Allocation |
| Largest Holdings [Text Block] | Top Ten Holdings Carlyle Group, Inc. 2.3 % Primo Brands Corporation 2.3 % e.l.f. Beauty, Inc. 2.1 % Doximity, Inc. 2.1 % Dynatrace, Inc. 2.0 % Talen Energy Corporation 2.0 % Confluent, Inc. 2.0 % SharkNinja, Inc. 2.0 % Bright Horizons Family Solutions, Inc. 1.8 % Chemed Corp. 1.8 % |
| William Blair Small-Mid Cap Growth Fund - Class R6 | |
| Shareholder Report [Line Items] | |
| Fund Name | William Blair Small-Mid CapGrowth Fund |
| Class Name | Class R6 |
| Trading Symbol | WSMRX |
| Annual or Semi-Annual Statement [Text Block] | This annual shareholder report contains important information about William Blair Small-Mid Cap Growth Fund Class R6 (the "Fund”) for the period of January 1, 2024, to December 31, 2024. |
| Shareholder Report Annual or Semi-Annual | annual shareholder report |
| Additional Information [Text Block] | You can find additional information about the Fund at https://www.williamblairfunds.com/literature/forms |
| Additional Information Phone Number | 1-800-742-7272 |
| Additional Information Website | https://www.williamblairfunds.com/literature/forms |
| Expenses [Text Block] | What were the Fund costs for the last year? (Based on a hypothetical $10,000 investment) Fund Cost of $10,000 Investment Cost of $10,000 Investment as a percentage Small-Mid Cap Growth Fund Class R6 $ 0.94 % |
| Expenses Paid, Amount | $ 99 |
| Expense Ratio, Percent | 0.94% |
| Factors Affecting Performance [Text Block] | Management’s Discussion of Fund Performance SUMMARY OF RESULTS The Fund returned 11.54% (net of fees) for the 12 months ended December 31, 2024. The Fund underperformed its benchmark, the Russell 2500 ™ ® The Fund lagged the return of the Russell 2500 Growth Index in the full year period, primarily driven by stock-specific dynamics. Stock selection in Energy, including our position in New Fortress, detracted from performance. Other notable detractors included Acadia Healthcare (Health Care), Evolent Health (Health Care), Neogen Corp (Health Care) and National Vision (Consumer Discretionary). Positive stock selection in Consumer Staples, including our positions in Freshpet and Primo Brands, added value during the period. Other top contributors included Doximity (Health Care), BWX Technologies (Industrials) and Insmed (Health Care). TOP PERFORMANCE CONTRIBUTORS Freshpet Doximity TOP PERFORMANCE DETRACTORS New Fortress Energy Acadia Healthcare |
| Performance Past Does Not Indicate Future [Text] | Keep in mind that the Fund’s past performance is not a good predictor of how the Fund will perform in the future. |
| Line Graph [Table Text Block] | |
| Average Annual Return [Table Text Block] | Average Annual Total Returns 1 Year 5 Years Since Inception Small-Mid Cap Growth Fund Class R6 11.54 % 7.86 % 8.40 % Russell 3000 ® 23.81 % 13.86 % 14.26 % Russell 2500 ™ 13.90 % 8.08 % 8.74 % |
| No Deduction of Taxes [Text Block] | The graph and table do not reflect the deduction of taxes that a shareholder would pay on Fund distributions or redemption of Fund shares. |
| Updated Performance Information Location [Text Block] | Visit www.williamblairfunds.com/funds/total-returns |
| Net Assets | $ 1,887,232,000 |
| Holdings Count | Holding | 80 |
| Advisory Fees Paid, Amount | $ 16,315,000 |
| Investment Company, Portfolio Turnover | 63% |
| Additional Fund Statistics [Text Block] | Key Fund Statistics Fund net assets (in $000s) $ Number of portfolio holdings 80 Net advisory fees paid (in $000s) $ Portfolio turnover rate 63 % |
| Holdings [Text Block] | Graphical Representation of Holdings The tables below show the investment makeup of the Fund, representing percentage of the total net assets of the Fund. Top Ten Holdings Carlyle Group, Inc. 2.3 % Primo Brands Corporation 2.3 % e.l.f. Beauty, Inc. 2.1 % Doximity, Inc. 2.1 % Dynatrace, Inc. 2.0 % Talen Energy Corporation 2.0 % Confluent, Inc. 2.0 % SharkNinja, Inc. 2.0 % Bright Horizons Family Solutions, Inc. 1.8 % Chemed Corp. 1.8 % Sector Allocation |
| Largest Holdings [Text Block] | Top Ten Holdings Carlyle Group, Inc. 2.3 % Primo Brands Corporation 2.3 % e.l.f. Beauty, Inc. 2.1 % Doximity, Inc. 2.1 % Dynatrace, Inc. 2.0 % Talen Energy Corporation 2.0 % Confluent, Inc. 2.0 % SharkNinja, Inc. 2.0 % Bright Horizons Family Solutions, Inc. 1.8 % Chemed Corp. 1.8 % |
| William Blair Small-Mid Cap Value Fund - Class I | |
| Shareholder Report [Line Items] | |
| Fund Name | William Blair Small-Mid CapValue Fund |
| Class Name | Class I |
| Trading Symbol | ISMVX |
| Annual or Semi-Annual Statement [Text Block] | This annual shareholder report contains important information about William Blair Small-Mid Cap Value Fund Class I (the "Fund”) for the period of January 1, 2024, to December 31, 2024. |
| Shareholder Report Annual or Semi-Annual | annual shareholder report |
| Additional Information [Text Block] | You can find additional information about the Fund at https://www.williamblairfunds.com/literature/forms |
| Additional Information Phone Number | 1-800-742-7272 |
| Additional Information Website | https://www.williamblairfunds.com/literature/forms |
| Expenses [Text Block] | What were the Fund costs for the last year? (Based on a hypothetical $10,000 investment) Fund Cost of $10,000 Investment Cost of $10,000 Investment as a percentage Small-Mid Cap Value Fund Class I $ 0.85 % |
| Expenses Paid, Amount | $ 87 |
| Expense Ratio, Percent | 0.85% |
| Factors Affecting Performance [Text Block] | Management’s Discussion of Fund Performance SUMMARY OF RESULTS The Fund returned 5.39% (net of fees) for the 12 months ended December 31, 2024. The Fund underperformed its benchmark, the Russell 2500 ® ® The Fund’s underperformance versus its benchmark was driven by poor sector stock selection in TOP PERFORMANCE CONTRIBUTORS Tecnoglass Alaska Air Group Armstrong World Industries TOP PERFORMANCE DETRACTORS QuidelOrtho Bloomin’ Brands Acadia Healthcare |
| Performance Past Does Not Indicate Future [Text] | Keep in mind that the Fund’s past performance is not a good predictor of how the Fund will perform in the future. |
| Line Graph [Table Text Block] | |
| Average Annual Return [Table Text Block] | Average Annual Total Returns 1 Year Since Inception Small-Mid Cap Value Fund Class I 5.39 % 8.72 % Russell 3000® Index 23.81 % 25.39 % Russell 2500® Value Index 10.98 % 16.09 % |
| No Deduction of Taxes [Text Block] | The graph and table do not reflect the deduction of taxes that a shareholder would pay on Fund distributions or redemption of Fund shares. |
| Updated Performance Information Location [Text Block] | Visit www.williamblairfunds.com/funds/total-returns |
| Net Assets | $ 3,350,000 |
| Holdings Count | Holding | 87 |
| Advisory Fees Paid, Amount | $ 0 |
| Investment Company, Portfolio Turnover | 44% |
| Additional Fund Statistics [Text Block] | Key Fund Statistics Fund net assets (in $000s) $ Number of portfolio holdings 87 Net advisory fees paid (in $000s) $ Portfolio turnover rate 44 % |
| Holdings [Text Block] | Graphical Representation of Holdings The tables below show the investment makeup of the Fund, representing percentage of the total net assets of the Fund. Top Ten Holdings Carlyle Group, Inc. 1.4 % Genpact Ltd. 1.4 % Columbia Banking System, Inc. 1.4 % Silgan Holdings, Inc. 1.4 % U.S. Foods Holding Corp. 1.4 % Labcorp Holdings, Inc. 1.4 % Tecnoglass, Inc. 1.4 % Air Lease Corp. 1.4 % Avnet, Inc. 1.4 % Maximus, Inc. 1.4 % Sector Allocation |
| Largest Holdings [Text Block] | Top Ten Holdings Carlyle Group, Inc. 1.4 % Genpact Ltd. 1.4 % Columbia Banking System, Inc. 1.4 % Silgan Holdings, Inc. 1.4 % U.S. Foods Holding Corp. 1.4 % Labcorp Holdings, Inc. 1.4 % Tecnoglass, Inc. 1.4 % Air Lease Corp. 1.4 % Avnet, Inc. 1.4 % Maximus, Inc. 1.4 % |
| William Blair Small-Mid Cap Value Fund - Class R6 | |
| Shareholder Report [Line Items] | |
| Fund Name | William Blair Small-Mid CapValue Fund |
| Class Name | Class R6 |
| Trading Symbol | RSMVX |
| Annual or Semi-Annual Statement [Text Block] | This annual shareholder report contains important information about William Blair Small-Mid Cap Value Fund Class R6 (the "Fund”) for the period of January 1, 2024, to December 31, 2024. |
| Shareholder Report Annual or Semi-Annual | annual shareholder report |
| Additional Information [Text Block] | You can find additional information about the Fund at https://www.williamblairfunds.com/literature/forms |
| Additional Information Phone Number | 1-800-742-7272 |
| Additional Information Website | https://www.williamblairfunds.com/literature/forms |
| Expenses [Text Block] | What were the Fund costs for the last year? (Based on a hypothetical $10,000 investment) Fund Cost of $10,000 Investment Cost of $10,000 Investment as a percentage Small-Mid Cap Value Fund Class R6 $ 0.80 % |
| Expenses Paid, Amount | $ 82 |
| Expense Ratio, Percent | 0.80% |
| Factors Affecting Performance [Text Block] | Management’s Discussion of Fund Performance SUMMARY OF RESULTS The Fund returned 5.37% (net of fees) for the 12 months ended December 31, 2024. The Fund underperformed its benchmark, the Russell 2500 ® ® The Fund’s underperformance versus its benchmark was driven by poor sector stock selection due to the Fund’s relative underperformance in the Consumer Discretionary, Information Technology, and Financials sectors. Despite negative performance, the Fund had positive sector allocation in TOP PERFORMANCE CONTRIBUTORS Tecnoglass Alaska Air Group Armstrong World Industries TOP PERFORMANCE DETRACTORS QuidelOrtho Bloomin’ Brands Acadia Healthcare |
| Performance Past Does Not Indicate Future [Text] | Keep in mind that the Fund’s past performance is not a good predictor of how the Fund will perform in the future. |
| Line Graph [Table Text Block] | |
| Average Annual Return [Table Text Block] | Average Annual Total Returns 1 Year Since Inception Small-Mid Cap Value Fund Class R6 5.37 % 8.74 % Russell 3000® Index 23.81 % 25.39 % Russell 2500® Value Index 10.98 % 16.09 % |
| No Deduction of Taxes [Text Block] | The graph and table do not reflect the deduction of taxes that a shareholder would pay on Fund distributions or redemption of Fund shares. |
| Updated Performance Information Location [Text Block] | Visit www.williamblairfunds.com/funds/total-returns |
| Net Assets | $ 3,350,000 |
| Holdings Count | Holding | 87 |
| Advisory Fees Paid, Amount | $ 0 |
| Investment Company, Portfolio Turnover | 44% |
| Additional Fund Statistics [Text Block] | Key Fund Statistics Fund net assets (in $000s) $ Number of portfolio holdings 87 Net advisory fees paid (in $000s) $ Portfolio turnover rate 44 % |
| Holdings [Text Block] | Graphical Representation of Holdings The tables below show the investment makeup of the Fund, representing percentage of the total net assets of the Fund. Top Ten Holdings Carlyle Group, Inc. 1.4 % Genpact Ltd. 1.4 % Columbia Banking System, Inc. 1.4 % Silgan Holdings, Inc. 1.4 % U.S. Foods Holding Corp. 1.4 % Labcorp Holdings, Inc. 1.4 % Tecnoglass, Inc. 1.4 % Air Lease Corp. 1.4 % Avnet, Inc. 1.4 % Maximus, Inc. 1.4 % Sector Allocation |
| Largest Holdings [Text Block] | Top Ten Holdings Carlyle Group, Inc. 1.4 % Genpact Ltd. 1.4 % Columbia Banking System, Inc. 1.4 % Silgan Holdings, Inc. 1.4 % U.S. Foods Holding Corp. 1.4 % Labcorp Holdings, Inc. 1.4 % Tecnoglass, Inc. 1.4 % Air Lease Corp. 1.4 % Avnet, Inc. 1.4 % Maximus, Inc. 1.4 % |
| William Blair Small Cap Growth Fund - Class 1 | |
| Shareholder Report [Line Items] | |
| Fund Name | William Blair Small Cap Growth Fund |
| Class Name | Class I |
| Trading Symbol | WBSIX |
| Annual or Semi-Annual Statement [Text Block] | This annual shareholder report contains important information about William Blair Small Cap Growth Fund Class I (the "Fund”) for the period of January 1, 2024, to December 31, 2024. |
| Shareholder Report Annual or Semi-Annual | annual shareholder report |
| Additional Information [Text Block] | You can find additional information about the Fund at https://www.williamblairfunds.com/literature/forms |
| Additional Information Phone Number | 1-800-742-7272 |
| Additional Information Website | https://www.williamblairfunds.com/literature/forms |
| Expenses [Text Block] | What were the Fund costs for the last year? (Based on a hypothetical $10,000 investment) Fund Cost of $10,000 Investment Cost of $10,000 Investment as a percentage Small Cap Growth Fund Class I $ 0.99 % |
| Expenses Paid, Amount | $ 109 |
| Expense Ratio, Percent | 0.99% |
| Factors Affecting Performance [Text Block] | Management’s Discussion of Fund Performance SUMMARY OF RESULTS The Fund returned 19.36% (net of fees) for the 12 months ended December 31, 2024. The Fund outperformed its benchmark, the Russell 2000 ® ® The Fund outperformed the return of the Russell 2000 Growth Index in the full year period, primarily driven by stock-specific dynamics. TOP PERFORMANCE CONTRIBUTORS Vital Farms Insmed TOP PERFORMANCE DETRACTORS New Fortress Energy Acadia Healthcare |
| Performance Past Does Not Indicate Future [Text] | Keep in mind that the Fund’s past performance is not a good predictor of how the Fund will perform in the future. |
| Line Graph [Table Text Block] | |
| Average Annual Return [Table Text Block] | Average Annual Total Returns 1 Year 5 Years 10 Years Small Cap Growth Fund Class I 19.36 % 11.38 % 11.57 % Russell 3000 ® 23.81 % 13.86 % 12.55 % Russell 2000 ® 15.15 % 6.86 % 8.09 % |
| No Deduction of Taxes [Text Block] | The graph and table do not reflect the deduction of taxes that a shareholder would pay on Fund distributions or redemption of Fund shares. |
| Updated Performance Information Location [Text Block] | Visit www.williamblairfunds.com/funds/total-returns |
| Net Assets | $ 917,371,000 |
| Holdings Count | Holding | 89 |
| Advisory Fees Paid, Amount | $ 6,841,000 |
| Investment Company, Portfolio Turnover | 75% |
| Additional Fund Statistics [Text Block] | Key Fund Statistics Fund net assets (in $000s) $ Number of portfolio holdings 89 Net advisory fees paid (in $000s) $ Portfolio turnover rate 75 % |
| Holdings [Text Block] | Graphical Representation of Holdings The tables below show the investment makeup of the Fund, representing percentage of the total net assets Top Ten Holdings Casella Waste Systems, Inc. 2.6 % Doximity, Inc. 1.9 % ACV Auctions, Inc. 1.9 % Mercury Systems, Inc. 1.7 % Brink's Co. 1.7 % Vericel Corp. 1.7 % FirstService Corp. 1.7 % Balchem Corp. 1.6 % FTAI Aviation Ltd. 1.6 % Planet Fitness, Inc. 1.5 % Sector Allocation |
| Largest Holdings [Text Block] | Top Ten Holdings Casella Waste Systems, Inc. 2.6 % Doximity, Inc. 1.9 % ACV Auctions, Inc. 1.9 % Mercury Systems, Inc. 1.7 % Brink's Co. 1.7 % Vericel Corp. 1.7 % FirstService Corp. 1.7 % Balchem Corp. 1.6 % FTAI Aviation Ltd. 1.6 % Planet Fitness, Inc. 1.5 % |
| William Blair Small Cap Growth Fund - Class N | |
| Shareholder Report [Line Items] | |
| Fund Name | William Blair Small Cap Growth Fund |
| Class Name | Class N |
| Trading Symbol | WBSNX |
| Annual or Semi-Annual Statement [Text Block] | This annual shareholder report contains important information about William Blair Small Cap Growth Fund Class N (the "Fund”) for the period of January 1, 2024, to December 31, 2024. |
| Shareholder Report Annual or Semi-Annual | annual shareholder report |
| Additional Information [Text Block] | You can find additional information about the Fund at https://www.williamblairfunds.com/literature/forms |
| Additional Information Phone Number | 1-800-742-7272 |
| Additional Information Website | https://www.williamblairfunds.com/literature/forms |
| Expenses [Text Block] | What were the Fund costs for the last year? (Based on a hypothetical $10,000 investment) Fund Cost of $10,000 Investment Cost of $10,000 Investment as a percentage Small Cap Growth Fund Class N $ 1.24 % |
| Expenses Paid, Amount | $ 136 |
| Expense Ratio, Percent | 1.24% |
| Factors Affecting Performance [Text Block] | Management’s Discussion of Fund Performance SUMMARY OF RESULTS The Fund returned 19.07% (net of fees) for the 12 months ended December 31, 2024. The Fund outperformed its benchmark, the Russell 2000 ® ® The Fund outperformed the return of the Russell 2000 Growth Index in the full year period, primarily driven by stock-specific dynamics. TOP PERFORMANCE CONTRIBUTORS Vital Farms Insmed TOP PERFORMANCE DETRACTORS New Fortress Energy Acadia Healthcare |
| Performance Past Does Not Indicate Future [Text] | Keep in mind that the Fund’s past performance is not a good predictor of how the Fund will perform in the future. |
| Line Graph [Table Text Block] | |
| Average Annual Return [Table Text Block] | Average Annual Total Returns 1 Year 5 Years 10 Years Small Cap Growth Fund Class N 19.07 % 11.10 % 11.29 % Russell 3000® Index 23.81 % 13.86 % 12.55 % Russell 2000® Growth Index 15.15 % 6.86 % 8.09 % |
| No Deduction of Taxes [Text Block] | The graph and table do not reflect the deduction of taxes that a shareholder would pay on Fund distributions |
| Updated Performance Information Location [Text Block] | Visit www.williamblairfunds.com/funds/total-returns |
| Net Assets | $ 917,371,000 |
| Holdings Count | Holding | 89 |
| Advisory Fees Paid, Amount | $ 6,841,000 |
| Investment Company, Portfolio Turnover | 75% |
| Additional Fund Statistics [Text Block] | Key Fund Statistics Fund net assets (in $000s) $ Number of portfolio holdings 89 Net advisory fees paid (in $000s) $ Portfolio turnover rate 75 % |
| Holdings [Text Block] | Graphical Representation of Holdings The tables below show the investment makeup of the Fund, representing Top Ten Holdings Casella Waste Systems, Inc. 2.6 % Doximity, Inc. 1.9 % ACV Auctions, Inc. 1.9 % Mercury Systems, Inc. 1.7 % Brink's Co. 1.7 % Vericel Corp. 1.7 % FirstService Corp. 1.7 % Balchem Corp. 1.6 % FTAI Aviation Ltd. 1.6 % Planet Fitness, Inc. 1.5 % Sector Allocation |
| Largest Holdings [Text Block] | Top Ten Holdings Casella Waste Systems, Inc. 2.6 % Doximity, Inc. 1.9 % ACV Auctions, Inc. 1.9 % Mercury Systems, Inc. 1.7 % Brink's Co. 1.7 % Vericel Corp. 1.7 % FirstService Corp. 1.7 % Balchem Corp. 1.6 % FTAI Aviation Ltd. 1.6 % Planet Fitness, Inc. 1.5 % |
| William Blair Small Cap Growth Fund - Class R6 | |
| Shareholder Report [Line Items] | |
| Fund Name | William Blair Small Cap Growth Fund |
| Class Name | Class R6 |
| Trading Symbol | WBSRX |
| Annual or Semi-Annual Statement [Text Block] | This annual shareholder report contains important information about William Blair Small Cap Growth Fund Class R6 (the "Fund”) for the period of January 1, 2024, to December 31, 2024. |
| Shareholder Report Annual or Semi-Annual | annual shareholder report |
| Additional Information [Text Block] | You can find additional information about the Fund at https://www.williamblairfunds.com/literature/forms |
| Additional Information Phone Number | 1-800-742-7272 |
| Additional Information Website | https://www.williamblairfunds.com/literature/forms |
| Expenses [Text Block] | What were the Fund costs for the last year? (Based on a hypothetical $10,000 investment) Fund Cost of $10,000 Investment Cost of $10,000 Investment as a percentage Small Cap Growth Fund Class R6 $ 0.94 % |
| Expenses Paid, Amount | $ 103 |
| Expense Ratio, Percent | 0.94% |
| Factors Affecting Performance [Text Block] | Management’s Discussion of Fund Performance SUMMARY OF RESULTS The Fund returned 19.41% (net of fees) for the 12 months ended December 31, 2024. The Fund outperformed its benchmark, the Russell 2000 ® ® The Fund outperformed the return of the Russell 2000 Growth Index in the full year period, primarily driven by stock-specific dynamics. TOP PERFORMANCE CONTRIBUTORS Vital Farms Insmed TOP PERFORMANCE DETRACTORS New Fortress Energy Acadia Healthcare |
| Performance Past Does Not Indicate Future [Text] | Keep in mind that the Fund’s past performance is not a good predictor of how the Fund will perform in the future. |
| Line Graph [Table Text Block] | |
| Average Annual Return [Table Text Block] | Average Annual Total Returns 1 Year 5 Years Since Inception Small Cap Growth Fund Class R6 19.41 % 11.45 % 10.56 % Russell 3000 ® 23.81 % 13.86 % 14.26 % Russell 2000 ® 15.15 % 6.86 % 7.40 % |
| No Deduction of Taxes [Text Block] | The graph and table do not reflect the deduction of taxes that a shareholder would pay on Fund distributions or redemption of Fund shares. |
| Updated Performance Information Location [Text Block] | Visit www.williamblairfunds.com/funds/total-returns |
| Net Assets | $ 917,371,000 |
| Holdings Count | Holding | 89 |
| Advisory Fees Paid, Amount | $ 6,841,000 |
| Investment Company, Portfolio Turnover | 75% |
| Additional Fund Statistics [Text Block] | Key Fund Statistics Fund net assets (in $000s) $ Number of portfolio holdings 89 Net advisory fees paid (in $000s) $ Portfolio turnover rate 75 % |
| Holdings [Text Block] | Graphical Representation of Holdings The tables below show the investment makeup of the Fund, representing percentage of the Top Ten Holdings Casella Waste Systems, Inc. 2.6 % Doximity, Inc. 1.9 % ACV Auctions, Inc. 1.9 % Mercury Systems, Inc. 1.7 % Brink's Co. 1.7 % Vericel Corp. 1.7 % FirstService Corp. 1.7 % Balchem Corp. 1.6 % FTAI Aviation Ltd. 1.6 % Planet Fitness, Inc. 1.5 % Sector Allocation |
| Largest Holdings [Text Block] | Top Ten Holdings Casella Waste Systems, Inc. 2.6 % Doximity, Inc. 1.9 % ACV Auctions, Inc. 1.9 % Mercury Systems, Inc. 1.7 % Brink's Co. 1.7 % Vericel Corp. 1.7 % FirstService Corp. 1.7 % Balchem Corp. 1.6 % FTAI Aviation Ltd. 1.6 % Planet Fitness, Inc. 1.5 % |
| William Blair Small Cap value fund - class I | |
| Shareholder Report [Line Items] | |
| Fund Name | William Blair Small Cap Value Fund |
| Class Name | Class I |
| Trading Symbol | ICSCX |
| Annual or Semi-Annual Statement [Text Block] | This annual shareholder report contains important information about William Blair Small Cap Value Fund Class I (the "Fund”) for the period of January 1, 2024, to December 31, 2024. |
| Shareholder Report Annual or Semi-Annual | annual shareholder report |
| Additional Information [Text Block] | You can find additional information about the Fund at https://www.williamblairfunds.com/literature/forms |
| Additional Information Phone Number | 1-800-742-7272 |
| Additional Information Website | https://www.williamblairfunds.com/literature/forms |
| Expenses [Text Block] | What were the Fund costs for the last year? (Based on a hypothetical $10,000 investment) Fund Cost of $10,000 Investment Cost of $10,000 Investment as a percentage Small Cap Value Fund Class I $ 0.89 % |
| Expenses Paid, Amount | $ 91 |
| Expense Ratio, Percent | 0.89% |
| Factors Affecting Performance [Text Block] | Management’s Discussion of Fund Performance SUMMARY OF RESULTS The Fund returned 3.44% (net of fees) for the 12 months ended December 31, 2024. The Fund underperformed its benchmark, the Russell 2000 ® ® The Fund’s underperformance versus its benchmark was driven by poor stock selection in the Consumer Discretionary, Materials, and Industrials sectors. Despite negative performance, the Fund had positive sector allocation in 7 of the 11 Global Industry Classification Standard economic sectors during the period. Positive sector allocation was especially additive to relative performance due to an underweight in the Health Care sector and an overweight position to the Information Technology sector. TOP PERFORMANCE CONTRIBUTORS Tecnoglass Tower Semiconductor Granite Construction TOP PERFORMANCE DETRACTORS Bloomin’ Brands Orion Janus International Group |
| Performance Past Does Not Indicate Future [Text] | Keep in mind that the Fund’s past performance is not a good predictor of how the Fund will perform in the future. |
| Line Graph [Table Text Block] | |
| Average Annual Return [Table Text Block] | Average Annual Total Returns 1 Year 5 Years 10 Years Small Cap Value Fund Class I 3.44 % 6.29 % 7.80 % Russell 3000® Index 23.81 % 13.86 % 12.55 % Russell 2000® Value Index 8.05 % 7.29 % 7.14 % |
| No Deduction of Taxes [Text Block] | The graph and table do not reflect the deduction of taxes that a shareholder would pay on Fund distributions or redemption of Fund shares. |
| Net Assets | $ 1,351,736,000 |
| Holdings Count | Holding | 110 |
| Advisory Fees Paid, Amount | $ 9,678,000 |
| Investment Company, Portfolio Turnover | 35% |
| Additional Fund Statistics [Text Block] | Key Fund Statistics Fund net assets (in $000s) $ Number of portfolio holdings 110 Net advisory fees paid (in $000s) $ Portfolio turnover rate 35 % |
| Holdings [Text Block] | Graphical Representation of Holdings The tables below show the investment makeup of the Fund, representing percentage of the total net assets of the Fund. Top Ten Holdings Tower Semiconductor Ltd. 1.5 % Ameris Bancorp 1.4 % Texas Capital Bancshares, Inc. 1.4 % Columbia Banking System, Inc. 1.4 % Taylor Morrison Home Corp. 1.4 % Gulfport Energy Corp. 1.3 % John Wiley & Sons, Inc. 1.3 % Old National Bancorp 1.3 % Knowles Corp. 1.3 % La-Z-Boy, Inc. 1.2 % Sector Allocation |
| Largest Holdings [Text Block] | Top Ten Holdings Tower Semiconductor Ltd. 1.5 % Ameris Bancorp 1.4 % Texas Capital Bancshares, Inc. 1.4 % Columbia Banking System, Inc. 1.4 % Taylor Morrison Home Corp. 1.4 % Gulfport Energy Corp. 1.3 % John Wiley & Sons, Inc. 1.3 % Old National Bancorp 1.3 % Knowles Corp. 1.3 % La-Z-Boy, Inc. 1.2 % |
| William Blair Small Cap value fund - class N | |
| Shareholder Report [Line Items] | |
| Fund Name | William Blair Small Cap Value Fund |
| Class Name | Class N |
| Trading Symbol | WBVNX |
| Annual or Semi-Annual Statement [Text Block] | This annual shareholder report contains important information about William Blair Small Cap Value Fund Class N (the "Fund”) for the period of January 1, 2024, to December 31, 2024. |
| Shareholder Report Annual or Semi-Annual | annual shareholder report |
| Additional Information [Text Block] | You can find additional information about the Fund at https://www.williamblairfunds.com/literature/forms |
| Additional Information Phone Number | 1-800-742-7272 |
| Additional Information Website | https://www.williamblairfunds.com/literature/forms |
| Expenses [Text Block] | What were the Fund costs for the last year? (Based on a hypothetical $10,000 investment) Fund Cost of $10,000 Investment Cost of $10,000 Investment as a percentage Small Cap Value Fund Class N $ 1.15 % |
| Expenses Paid, Amount | $ 117 |
| Expense Ratio, Percent | 1.15% |
| Factors Affecting Performance [Text Block] | Management’s Discussion of Fund Performance SUMMARY OF RESULTS The Fund returned 3.21% (net of fees) for the 12 months ended December 31, 2024. The Fund underperformed its benchmark, the Russell 2000 ® ® The Fund’s TOP PERFORMANCE CONTRIBUTORS Tecnoglass Tower Semiconductor Granite Construction TOP PERFORMANCE DETRACTORS Bloomin’ Brands Orion Janus International Group |
| Performance Past Does Not Indicate Future [Text] | Keep in mind that the Fund’s past performance is not a good predictor of how the Fund will perform in the future. |
| Line Graph [Table Text Block] | |
| Average Annual Return [Table Text Block] | Average Annual Total Returns 1 Year Since Inception Small Cap Value Fund Class N 3.21 % 2.76 % Russell 3000 ® 23.81 % 9.71 % Russell 2000 ® 8.05 % 3.81 % |
| No Deduction of Taxes [Text Block] | The graph and table do not reflect the deduction of taxes that a shareholder would pay on Fund distributions or redemption of Fund shares. |
| Updated Performance Information Location [Text Block] | Visit www.williamblairfunds.com/funds/total-returns |
| Net Assets | $ 1,351,736,000 |
| Holdings Count | Holding | 110 |
| Advisory Fees Paid, Amount | $ 9,678,000 |
| Investment Company, Portfolio Turnover | 35% |
| Additional Fund Statistics [Text Block] | Key Fund Fund net assets (in $000s) $ Number of portfolio holdings 110 Net advisory fees paid (in $000s) $ Portfolio turnover rate 35 % |
| Holdings [Text Block] | Graphical Representation of Holdings The tables below show the investment makeup of the Fund, representing percentage of the total net assets of the Fund. Top Ten Holdings Tower Semiconductor Ltd. 1.5 % Ameris Bancorp 1.4 % Texas Capital Bancshares, Inc. 1.4 % Columbia Banking System, Inc. 1.4 % Taylor Morrison Home Corp. 1.4 % Gulfport Energy Corp. 1.3 % John Wiley & Sons, Inc. 1.3 % Old National Bancorp 1.3 % Knowles Corp. 1.3 % La-Z-Boy, Inc. 1.2 % Sector Allocation |
| Largest Holdings [Text Block] | Top Ten Holdings Tower Semiconductor Ltd. 1.5 % Ameris Bancorp 1.4 % Texas Capital Bancshares, Inc. 1.4 % Columbia Banking System, Inc. 1.4 % Taylor Morrison Home Corp. 1.4 % Gulfport Energy Corp. 1.3 % John Wiley & Sons, Inc. 1.3 % Old National Bancorp 1.3 % Knowles Corp. 1.3 % La-Z-Boy, Inc. 1.2 % |
| William Blair Small Cap Value Fund - Class R6 [Member] | |
| Shareholder Report [Line Items] | |
| Fund Name | William Blair Small Cap Value Fund |
| Class Name | Class R6 |
| Trading Symbol | WBVRX |
| Annual or Semi-Annual Statement [Text Block] | This annual shareholder report contains important information about William Blair Small Cap Value Fund Class R6 (the "Fund”) for the period of January 1, 2024, to December 31, 2024. |
| Shareholder Report Annual or Semi-Annual | annual shareholder report |
| Additional Information [Text Block] | You can find additional information about the Fund at https://www.williamblairfunds.com/literature/forms |
| Additional Information Phone Number | 1-800-742-7272 |
| Additional Information Website | https://www.williamblairfunds.com/literature/forms |
| Expenses [Text Block] | What were the Fund costs for the last year? (Based on a hypothetical $10,000 investment) Fund Cost of $10,000 Investment Cost of $10,000 Investment as a percentage Small Cap Value Fund Class R6 $ 0.79 % |
| Expenses Paid, Amount | $ 80 |
| Expense Ratio, Percent | 0.79% |
| Factors Affecting Performance [Text Block] | Management’s Discussion of Fund Performance SUMMARY OF RESULTS The Fund returned 3.56% (net of fees) for the 12 months ended December 31, 2024. The Fund underperformed its benchmark, the Russell 2000 ® ® TOP PERFORMANCE CONTRIBUTORS Tecnoglass Tower Semiconductor Granite Construction TOP PERFORMANCE DETRACTORS Bloomin’ Brands Orion Janus International Group |
| Performance Past Does Not Indicate Future [Text] | Keep in mind that the Fund’s past performance is not a good predictor of how the Fund will perform in the future. |
| Line Graph [Table Text Block] | |
| Average Annual Return [Table Text Block] | Average Annual Total Returns 1 Year Since Inception Small Cap Value Fund Class R6 3.56 % 3.12 % Russell 3000® Index 23.81 % 9.71 % Russell 2000® Value Index 8.05 % 3.81 % |
| No Deduction of Taxes [Text Block] | The graph and table do not reflect the deduction of taxes that a shareholder would pay on Fund distributions or redemption of Fund shares. |
| Updated Performance Information Location [Text Block] | Visit www.williamblairfunds.com/funds/total-returns |
| Net Assets | $ 1,351,736,000 |
| Holdings Count | Holding | 110 |
| Advisory Fees Paid, Amount | $ 9,678,000 |
| Investment Company, Portfolio Turnover | 35% |
| Additional Fund Statistics [Text Block] | Key Fund Statistics Fund net assets (in $000s) $ Number of portfolio holdings 110 Net advisory fees paid (in $000s) $ Portfolio turnover rate 35 % |
| Holdings [Text Block] | Graphical Representation of Holdings The tables below Top Ten Holdings Tower Semiconductor Ltd. 1.5 % Ameris Bancorp 1.4 % Texas Capital Bancshares, Inc. 1.4 % Columbia Banking System, Inc. 1.4 % Taylor Morrison Home Corp. 1.4 % Gulfport Energy Corp. 1.3 % John Wiley & Sons, Inc. 1.3 % Old National Bancorp 1.3 % Knowles Corp. 1.3 % La-Z-Boy, Inc. 1.2 % Sector Allocation |
| Largest Holdings [Text Block] | Top Ten Holdings Tower Semiconductor Ltd. 1.5 % Ameris Bancorp 1.4 % Texas Capital Bancshares, Inc. 1.4 % Columbia Banking System, Inc. 1.4 % Taylor Morrison Home Corp. 1.4 % Gulfport Energy Corp. 1.3 % John Wiley & Sons, Inc. 1.3 % Old National Bancorp 1.3 % Knowles Corp. 1.3 % La-Z-Boy, Inc. 1.2 % |
| William Blair Global Leaders Fund - Class I | |
| Shareholder Report [Line Items] | |
| Fund Name | William Blair Global Leaders Fund |
| Class Name | Class I |
| Trading Symbol | WGFIX |
| Annual or Semi-Annual Statement [Text Block] | This annual shareholder report contains important information about William Blair Global Leaders Fund Class I (the "Fund”) for the period of January 1, 2024, to December 31, 2024. |
| Shareholder Report Annual or Semi-Annual | annual shareholder report |
| Additional Information [Text Block] | You can find additional information about the Fund at https://www.williamblairfunds.com/literature/forms |
| Material Fund Change Notice [Text Block] | This report describes changes to the Fund that occurred during the reporting period. |
| Additional Information Phone Number | 1-800-742-7272 |
| Additional Information Website | https://www.williamblairfunds.com/literature/forms |
| Expenses [Text Block] | What were the Fund costs for the last year? (Based on a hypothetical $10,000 investment) Fund Cost of $10,000 Investment Cost of $10,000 Investment as a percentage Global Leaders Fund Class I $ 0.90 % |
| Expenses Paid, Amount | $ 93 |
| Expense Ratio, Percent | 0.90% |
| Factors Affecting Performance [Text Block] | Management’s Discussion of Fund Performance SUMMARY OF RESULTS The Fund returned 7.71% (net of fees) for the 12 months ended December 31, 2024. The Fund underperformed All Underperformance versus the Fund's benchmark was driven by stock selection across sectors and in the United States. Selections in Information Technology (“IT”), Industrials, and Health Care were the primary detractors. While the overweight to IT was additive, negative selection was driven by a net underweight position in NVIDIA as it was purchased midyear and the omission of Apple. Within Industrials, Old Dominion Freight Line and Atlas Copco were the primary detractors, while Icon weighed on Health Care returns. TOP PERFORMANCE DETRACTORS NVIDIA Within Industrials, both Old Dominion Freight Line Atlas Copco Negative stock selection in Health Care was driven by exposure to Icon TOP PERFORMANCE CONTRIBUTORS Partly offsetting underperformance was a combination of sector allocation driven by an overweight to IT and underweight to Consumer Staples, and stock selection in Communication Services and emerging markets. Within Communication Services, positions in Meta Platforms Alphabet the In Consumer Staples, relative outperformance was driven by an underweight to an underperforming sector and exposure to Walmart Within emerging markets, relative performance was partially offset by a meaningful position in Taiwan Semiconductor Manufacturing Company |
| Performance Past Does Not Indicate Future [Text] | Keep in mind that the Fund’s past performance is not a good predictor of how the Fund will perform in the future. |
| Line Graph [Table Text Block] | |
| Average Annual Return [Table Text Block] | Average Annual Total Returns 1 Year 5 Years 10 Years Global Leaders Fund Class I 7.71 % 7.60 % 9.03 % MSCI All Country World IMI (net) 16.37 % 9.67 % 9.00 % |
| No Deduction of Taxes [Text Block] | The graph and table do not reflect the deduction of taxes that a shareholder would pay on Fund distributions or redemption of Fund shares. |
| Material Change Date | Dec. 31, 2023 |
| Updated Performance Information Location [Text Block] | Visit www.williamblairfunds.com/funds/total-returns |
| Net Assets | $ 54,177,000 |
| Holdings Count | Holding | 69 |
| Advisory Fees Paid, Amount | $ 493,000 |
| Investment Company, Portfolio Turnover | 57% |
| Additional Fund Statistics [Text Block] | Key Fund Statistics Fund net assets (in $000s) $ Number of portfolio holdings 69 Net advisory fees paid (in $000s) $ Portfolio turnover rate 57 % |
| Holdings [Text Block] | Graphical Representation of Holdings The tables below show the investment makeup of the Fund, representing percentage of the total net assets of the Fund. Top Ten Holdings Microsoft Corp. 5.7 % Amazon.com, Inc. 4.7 % NVIDIA Corp. 4.4 % Alphabet, Inc. 3.9 % Taiwan Semiconductor Manufacturing Co. Ltd. 3.3 % Mastercard, Inc. 3.2 % Meta Platforms, Inc. 3.0 % Compass Group PLC 2.1 % Hermes International SCA 2.1 % Broadcom, Inc. 2.1 % Sector Allocation Geographic Diversification |
| Largest Holdings [Text Block] | Top Ten Holdings Microsoft Corp. 5.7 % Amazon.com, Inc. 4.7 % NVIDIA Corp. 4.4 % Alphabet, Inc. 3.9 % Taiwan Semiconductor Manufacturing Co. Ltd. 3.3 % Mastercard, Inc. 3.2 % Meta Platforms, Inc. 3.0 % Compass Group PLC 2.1 % Hermes International SCA 2.1 % Broadcom, Inc. 2.1 % |
| Material Fund Change [Text Block] | Material Fund Changes Effective September 9, 2024, the Fund's principal investment strategies were revised to provide that the Fund’s investments in companies located outside the United States may be lower than 40% of the Fund’s assets if deemed appropriate by the Fund’s investment adviser, but such investments may not be lower than 30% of the Fund’s assets. In addition, the restriction on investing more than 65% of the Fund’s equity holdings in securities of issuers in any one country at any given time was eliminated. This is a summary of certain changes to the Fund since December 31, 2023. For more complete information, you may review the Fund's next prospectus, which we expect will be available by May 1, 2025 at www.williamblairfunds.com/literature/forms |
| Material Fund Change Strategies [Text Block] | Effective September 9, 2024, the Fund's principal investment strategies were revised to provide that the Fund’s investments in companies located outside the United States may be lower than 40% of the Fund’s assets if deemed appropriate by the Fund’s investment adviser, but such investments may not be lower than 30% of the Fund’s assets. In addition, the restriction on investing more than 65% of the Fund’s equity holdings in securities of issuers in any one country at any given time was eliminated. |
| Summary of Change Legend [Text Block] | This is a summary of certain changes to the Fund since December 31, 2023. For more complete information, you may review the Fund's next prospectus, which we expect will be available by May 1, 2025 at www.williamblairfunds.com/literature/forms |
| Updated Prospectus Phone Number | 1-800-742-7272 |
| Updated Prospectus Web Address | www.williamblairfunds.com/literature/forms |
| William Blair Global Leaders Fund - Class N | |
| Shareholder Report [Line Items] | |
| Fund Name | William Blair Global Leaders Fund |
| Class Name | Class N |
| Trading Symbol | WGGNX |
| Annual or Semi-Annual Statement [Text Block] | This annual shareholder report contains important information about William Blair Global Leaders Fund Class N (the "Fund”) for the period of January 1, 2024, to December 31, 2024. |
| Shareholder Report Annual or Semi-Annual | annual shareholder report |
| Additional Information [Text Block] | You can find additional information about the Fund at https://www.williamblairfunds.com/literature/forms |
| Material Fund Change Notice [Text Block] | This report describes changes to the Fund that occurred during the reporting period. |
| Additional Information Phone Number | 1-800-742-7272 |
| Additional Information Website | https://www.williamblairfunds.com/literature/forms |
| Expenses [Text Block] | What were the Fund costs for the last year? (Based on a hypothetical $10,000 investment) Fund Cost of $10,000 Investment Cost of $10,000 Investment as a percentage Global Leaders Fund Class N $ 1.15 % |
| Expenses Paid, Amount | $ 119 |
| Expense Ratio, Percent | 1.15% |
| Factors Affecting Performance [Text Block] | Management’s Discussion of Fund Performance SUMMARY OF RESULTS The Fund returned 7.47% (net of fees) for the 12 months ended December 31, 2024. The Fund underperformed its benchmark, the MSCI All Underperformance versus the Fund's benchmark was driven by stock selection across sectors and in the United States. Selections in Information Technology (“IT”), Industrials, and Health Care were the primary detractors. While the overweight to IT was additive, negative selection was driven by a net underweight position in NVIDIA as it was purchased midyear and the omission of Apple. Within Industrials, Old Dominion Freight Line and Atlas Copco were the primary detractors, while Icon weighed on Health Care returns. TOP PERFORMANCE DETRACTORS NVIDIA Within Industrials, both Old Dominion Freight Line Atlas Copco Negative stock selection in Health Care was driven by exposure to Icon TOP PERFORMANCE CONTRIBUTORS Partly offsetting underperformance was a combination of sector allocation driven by an overweight to IT and underweight to Consumer Staples, and stock selection in Communication Services and emerging markets. Within Communication Services, positions in Meta Platforms Alphabet the In Consumer Staples, relative outperformance was driven by an underweight to an underperforming sector and exposure to Walmart Within emerging markets, relative performance was partially offset by a meaningful position in Taiwan Semiconductor Manufacturing Company |
| Performance Past Does Not Indicate Future [Text] | Keep in mind that the Fund’s past performance is not a good predictor of how the Fund will perform in the future. |
| Line Graph [Table Text Block] | |
| Average Annual Return [Table Text Block] | Average Annual Total Returns 1 Year 5 Years 10 Years Global Leaders Fund Class N 7.47 % 7.34 % 8.74 % MSCI All Country World IMI (net) 16.37 % 9.67 % 9.00 % |
| No Deduction of Taxes [Text Block] | The graph and table do not reflect the deduction of taxes that a shareholder would pay on Fund distributions or redemption of Fund shares. |
| Material Change Date | Dec. 31, 2023 |
| Updated Performance Information Location [Text Block] | Visit www.williamblairfunds.com/funds/total-returns |
| Net Assets | $ 54,177,000 |
| Holdings Count | Holding | 69 |
| Advisory Fees Paid, Amount | $ 493,000 |
| Investment Company, Portfolio Turnover | 57% |
| Additional Fund Statistics [Text Block] | Key Fund Statistics Fund net assets (in $000s) $ Number of portfolio holdings 69 Net advisory fees paid (in $000s) $ Portfolio turnover rate 57 % |
| Holdings [Text Block] | Graphical Representation of Holdings The tables below show the investment makeup of the Fund, representing percentage of the total net assets of the Fund. Top Ten Holdings Microsoft Corp. 5.7 % Amazon.com, Inc. 4.7 % NVIDIA Corp. 4.4 % Alphabet, Inc. 3.9 % Taiwan Semiconductor Manufacturing Co. Ltd. 3.3 % Mastercard, Inc. 3.2 % Meta Platforms, Inc. 3.0 % Compass Group PLC 2.1 % Hermes International SCA 2.1 % Broadcom, Inc. 2.1 % Sector Allocation Geographic Diversification |
| Largest Holdings [Text Block] | Top Ten Holdings Microsoft Corp. 5.7 % Amazon.com, Inc. 4.7 % NVIDIA Corp. 4.4 % Alphabet, Inc. 3.9 % Taiwan Semiconductor Manufacturing Co. Ltd. 3.3 % Mastercard, Inc. 3.2 % Meta Platforms, Inc. 3.0 % Compass Group PLC 2.1 % Hermes International SCA 2.1 % Broadcom, Inc. 2.1 % |
| Material Fund Change [Text Block] | Material Fund Changes Effective September 9, 2024, the Fund's principal investment strategies were revised to provide that the Fund’s investments in companies located outside the United States may be lower than 40% of the Fund’s assets if deemed appropriate by the Fund’s investment adviser, but such investments may not be lower than 30% of the Fund’s assets. In addition, the restriction on investing more than 65% of the Fund’s equity holdings in securities of issuers in any one country at any given time was eliminated. This is a summary of certain changes to the Fund since December 31, 2023. For more complete information, you may review the Fund's next prospectus, which we expect will be available by May 1, 2025 at www.williamblairfunds.com/literature/forms |
| Material Fund Change Strategies [Text Block] | Effective September 9, 2024, the Fund's principal investment strategies were revised to provide that the Fund’s investments in companies located outside the United States may be lower than 40% of the Fund’s assets if deemed appropriate by the Fund’s investment adviser, but such investments may not be lower than 30% of the Fund’s assets. In addition, the restriction on investing more than 65% of the Fund’s equity holdings in securities of issuers in any one country at any given time was eliminated. |
| Summary of Change Legend [Text Block] | This is a summary of certain changes to the Fund since December 31, 2023. For more complete information, you may review the Fund's next prospectus, which we expect will be available by May 1, 2025 at www.williamblairfunds.com/literature/forms |
| Updated Prospectus Phone Number | 1-800-742-7272 |
| Updated Prospectus Web Address | www.williamblairfunds.com/literature/forms |
| William Blair Global Leaders Fund - Class R6 | |
| Shareholder Report [Line Items] | |
| Fund Name | William Blair Global Leaders Fund |
| Class Name | Class R6 |
| Trading Symbol | BGGIX |
| Annual or Semi-Annual Statement [Text Block] | This annual shareholder report contains important information about William Blair Global Leaders Fund Class R6 (the "Fund”) for the period of January 1, 2024, to December 31, 2024. |
| Shareholder Report Annual or Semi-Annual | annual shareholder report |
| Additional Information [Text Block] | You can find additional information about the Fund at https://www.williamblairfunds.com/literature/forms |
| Material Fund Change Notice [Text Block] | This report describes changes to the Fund that occurred during the reporting period. |
| Additional Information Phone Number | 1-800-742-7272 |
| Additional Information Website | https://www.williamblairfunds.com/literature/forms |
| Expenses [Text Block] | What were the Fund costs for the last year? (Based on a hypothetical $10,000 investment) Fund Cost of $10,000 Investment Cost of $10,000 Investment as a percentage Global Leaders Fund Class R6 $ 0.85 % |
| Expenses Paid, Amount | $ 88 |
| Expense Ratio, Percent | 0.85% |
| Factors Affecting Performance [Text Block] | Management’s Discussion of Fund Performance SUMMARY OF RESULTS The Fund returned 7.78% (net of fees) for the 12 months ended December 31, 2024. The Fund underperformed its benchmark, the MSCI A ll Underperformance versus the Fund's benchmark was driven by stock selection across sectors and in the United States. Selections in Information Technology (“IT”), Industrials, and Health Care were the primary detractors. While the overweight to IT was additive, negative selection was driven by a net underweight position in NVIDIA as it was purchased midyear and the omission of Apple. Within Industrials, Old Dominion Freight Line and Atlas Copco were the primary detractors, while Icon weighed on Health Care returns. TOP PERFORMANCE DETRACTORS NVIDIA Within Industrials, both Old Dominion Freight Line Atlas Copco Negative stock selection in Health Care was driven by exposure to Icon TOP PERFORMANCE CONTRIBUTORS Partly offsetting underperformance was a combination of sector allocation driven by an overweight to IT and underweight to Consumer Staples, and stock selection in Communication Services and emerging markets. Within Communication Services, positions in Meta Platforms Alphabet the In Consumer Staples, relative outperformance was driven by an underweight to an underperforming sector and exposure to Walmart Within emerging markets, relative performance was partially offset by a meaningful position in Taiwan Semiconductor Manufacturing Company |
| Performance Past Does Not Indicate Future [Text] | Keep in mind that the Fund’s past performance is not a good predictor of how the Fund will perform in the future. |
| Line Graph [Table Text Block] | |
| Average Annual Return [Table Text Block] | Average Annual Total Returns 1 Year 5 Years 10 Years Global Leaders Fund Class R6 7.78 % 7.66 % 9.09 % MSCI All Country World IMI (net) 16.37 % 9.67 % 9.00 % |
| No Deduction of Taxes [Text Block] | The graph and table do not reflect the deduction of taxes that a shareholder would pay on Fund distributions or redemption of Fund shares. |
| Material Change Date | Dec. 31, 2023 |
| Updated Performance Information Location [Text Block] | Visit www.williamblairfunds.com/funds/total-returns |
| Net Assets | $ 54,177,000 |
| Holdings Count | Holding | 69 |
| Advisory Fees Paid, Amount | $ 493,000 |
| Investment Company, Portfolio Turnover | 57% |
| Additional Fund Statistics [Text Block] | Key Fund Statistics Fund net assets (in $000s) $ Number of portfolio holdings 69 Net advisory fees paid (in $000s) $ Portfolio turnover rate 57 % |
| Holdings [Text Block] | Graphical Representation of Holdings The tables below show the investment makeup of the Fund, representing percentage of the total net assets of the Fund. Top Ten Holdings Microsoft Corp. 5.7 % Amazon.com, Inc. 4.7 % NVIDIA Corp. 4.4 % Alphabet, Inc. 3.9 % Taiwan Semiconductor Manufacturing Co. Ltd. 3.3 % Mastercard, Inc. 3.2 % Meta Platforms, Inc. 3.0 % Compass Group PLC 2.1 % Hermes International SCA 2.1 % Broadcom, Inc. 2.1 % Sector Allocation Geographic Diversification |
| Largest Holdings [Text Block] | Top Ten Holdings Microsoft Corp. 5.7 % Amazon.com, Inc. 4.7 % NVIDIA Corp. 4.4 % Alphabet, Inc. 3.9 % Taiwan Semiconductor Manufacturing Co. Ltd. 3.3 % Mastercard, Inc. 3.2 % Meta Platforms, Inc. 3.0 % Compass Group PLC 2.1 % Hermes International SCA 2.1 % Broadcom, Inc. 2.1 % |
| Material Fund Change [Text Block] | Material Fund Changes Effective September 9, 2024, the Fund's principal investment strategies were revised to provide that the Fund’s investments in companies located outside the United States may be lower than 40% of the Fund’s assets if deemed appropriate by the Fund’s investment adviser, but such investments may not be lower than 30% of the Fund’s assets. In addition, the restriction on investing more than 65% of the Fund’s equity holdings in securities of issuers in any one country at any given time was eliminated. This is a summary of certain changes to the Fund since December 31, 2023. For more complete information, you may review the Fund's next prospectus, which we expect will be available by May 1, 2025 at www.williamblairfunds.com/literature/forms |
| Material Fund Change Strategies [Text Block] | Effective September 9, 2024, the Fund's principal investment strategies were revised to provide that the Fund’s investments in companies located outside the United States may be lower than 40% of the Fund’s assets if deemed appropriate by the Fund’s investment adviser, but such investments may not be lower than 30% of the Fund’s assets. In addition, the restriction on investing more than 65% of the Fund’s equity holdings in securities of issuers in any one country at any given time was eliminated. |
| Summary of Change Legend [Text Block] | This is a summary of certain changes to the Fund since December 31, 2023. For more complete information, you may review the Fund's next prospectus, which we expect will be available by May 1, 2025 at www.williamblairfunds.com/literature/forms |
| Updated Prospectus Phone Number | 1-800-742-7272 |
| Updated Prospectus Web Address | www.williamblairfunds.com/literature/forms |
| William Blair International Leaders Fund - Class I | |
| Shareholder Report [Line Items] | |
| Fund Name | William Blair InternationalLeaders Fund |
| Class Name | Class I |
| Trading Symbol | WILIX |
| Annual or Semi-Annual Statement [Text Block] | This annual shareholder report contains important information about William Blair International Leaders Fund Class I (the "Fund”) for the period of January 1, 2024, to December 31, 2024. |
| Shareholder Report Annual or Semi-Annual | annual shareholder report |
| Additional Information [Text Block] | You can find additional information about the Fund at https://www.williamblairfunds.com/literature/forms |
| Additional Information Phone Number | 1-800-742-7272 |
| Additional Information Website | https://www.williamblairfunds.com/literature/forms |
| Expenses [Text Block] | What were the Fund costs for the last year? (Based on a hypothetical $10,000 investment) Fund Cost of $10,000 Investment Cost of $10,000 Investment as a percentage International Leaders Fund Class I $ 0.90 % |
| Expenses Paid, Amount | $ 90 |
| Expense Ratio, Percent | 0.90% |
| Factors Affecting Performance [Text Block] | Management’s Discussion of Fund Performance SUMMARY OF RESULTS The Fund returned -0.50% (net of fees) for the 12 months ended December 31, 2024. The Fund underperformed its benchmark, the MSCI All Country World ex-U.S. IMI (net), which returned 5.23%. The Fund's benchmark is also a broad measure of market performance. Underperformance versus the benchmark was driven by stock selection on both a sector and regional view. From a sector perspective, allocation effect was positive due to underweight exposures to Materials and Consumer Staples and overweight exposures to Information Technology (“IT”) and Industrials. This was partially offset by an underweight to Financials. Selection effect was negative primarily within Industrials, IT, and Financials. On a regional view, allocation effect was negative due to an underweight to China and overweight in Europe. Selection effect was negative primarily in Europ e TOP PERFORMANCE DETRACTORS The largest contributor to negative performance came from Financials and is due to a combination of the underweight to the sector, specifically underweight to banks and selection within banks. The industry did particularly well due to fewer rate cuts than initially anticipated and a better economic environment than anticipated. Additionally, the strategy was negatively impacted by its position in Itau Unibanco Selection within Industrials was also negative and driven in part by positions in Vinci Kingsp a Atlas Copco Kingspan has developed a differentiated technology within the building materials industry that may continue to gain market share globally given the strong value proposition it offers customers. The stock underperformed in the fourth quarter after forward guidance called for the company’s full year 2024 to be “broadly inline” with 2023, after previously guiding for growth. Atlas Copco manufactures industrial equipment for ind u s Within IT, the strategy benefited from being overweight semiconductors, but selection within semiconductors was negative due in part to positions in Lasertec Infineon Infineon specializes in automotive semiconductors and the r n TOP PERFORMANCE CONTRIBUTORS Partially offsetting this was strong performance from Taiwan Semiconductor Manufacturing Company |
| Performance Past Does Not Indicate Future [Text] | Keep in mind that the Fund’s past performance is not a good predictor of how the Fund will perform in the future. |
| Line Graph [Table Text Block] | |
| Average Annual Return [Table Text Block] | Average Annual Total Returns 1 Year 5 Years 10 Years International Leaders Fund Class I -0.50 % 2.35 % 6.10 % MSCI All Country World ex-U.S. IMI (net) 5.23 % 4.12 % 4.91 % |
| No Deduction of Taxes [Text Block] | The graph and table do not reflect the deduction of taxes that a shareholder would pay on Fund distributions or redemption of Fund shares. |
| Updated Performance Information Location [Text Block] | Visit www.williamblairfunds.com/funds/total-returns |
| Net Assets | $ 1,160,666,000 |
| Holdings Count | Holding | 66 |
| Advisory Fees Paid, Amount | $ 8,632,000 |
| Investment Company, Portfolio Turnover | 66% |
| Additional Fund Statistics [Text Block] | Key Fund Statistics Fund net assets (in $000 s $ Number of portfolio holdings 66 Net advisory fees paid (in $000s) $ Portfolio turnover rate 66 % |
| Holdings [Text Block] | Graphical Representation of Holdings The tables below show the investment makeup of the Fund, representing percenta ge nd Top Ten Holdings Taiwan Semiconductor Manufacturing Co. Ltd. 4.3 % SAP SE 2.6 % MediaTek, Inc. 2.3 % Schneider Electric SE 2.2 % Industria de Diseno Textil SA 2.2 % London Stock Exchange Group PLC 2.1 % Constellation Software, Inc. 2.1 % Compass Group PLC 2.0 % Sandoz Group AG 2.0 % Intact Financial Corp. 2.0 % Sector Allocation Geographic Diversification |
| Largest Holdings [Text Block] | Top Ten Holdings Taiwan Semiconductor Manufacturing Co. Ltd. 4.3 % SAP SE 2.6 % MediaTek, Inc. 2.3 % Schneider Electric SE 2.2 % Industria de Diseno Textil SA 2.2 % London Stock Exchange Group PLC 2.1 % Constellation Software, Inc. 2.1 % Compass Group PLC 2.0 % Sandoz Group AG 2.0 % Intact Financial Corp. 2.0 % |
| William Blair International Leaders Fund - Class N | |
| Shareholder Report [Line Items] | |
| Fund Name | William Blair InternationalLeaders Fund |
| Class Name | Class N |
| Trading Symbol | WILNX |
| Annual or Semi-Annual Statement [Text Block] | This annual shareholder report contains important information about William Blair International Leaders Fund Class N (the "Fund”) for the period of January 1, 2024, to December 31, 2024. |
| Shareholder Report Annual or Semi-Annual | annual shareholder report |
| Additional Information [Text Block] | You can find additional information about the Fund at https://www.williamblairfunds.com/literature/forms |
| Additional Information Phone Number | 1-800-742-7272 |
| Additional Information Website | https://www.williamblairfunds.com/literature/forms |
| Expenses [Text Block] | What were the Fund costs for the last year? (Based on a hypothetical $10,000 investment) Fund Cost of $10,000 Investment Cost of $10,000 Investment as a percentage International Leaders Fund Class N $ 1.15 % |
| Expenses Paid, Amount | $ 115 |
| Expense Ratio, Percent | 1.15% |
| Factors Affecting Performance [Text Block] | Management’s Discussion of Fund Performance SUMMARY OF RESULTS The Fund returned -0.76% (net of fees) for the 12 months ended December 31, 2024. The Fund underperformed its benchmark, the MSCI All Country World ex-U.S. IMI (net), which returned 5.23%. The Fund's benchmark is also a broad measure of market performance. Underperformance versus the benchmark was driven by stock selection on both a sector and regional view. From a sector perspective, allocation effect was positive due to underweight exposures to Materials and Consumer Staples and overweight exposures to Information Technology (“IT”) and Industrials. This was partially offset by an underweight to Financials. Selection effect was negative primarily within Industrials, IT, and Financials. On a regional view, allocatio n TOP PERFORMANCE DETRACTORS The largest contributor to negative performance came from Financials and is due to a combination of the underweight to the sector, specifically underweight to banks and selection within banks. The industry did particularly well due to fewer rate cuts than initially anticipated and a better economic environment than anticipated. Additionally, the strategy was negatively impacted by its position in Itau Unibanco Selection within Industrials was also negative and driven in part by positions in Vinci Kingspan Atlas Copco Kingspan has developed a differentiated technology within the building materials industry that may continue to gain market share globally given the strong value proposition it offers customers. The stock underperformed in the fourth quarter after forward guidance called for the company’s full year 2024 to be “broadly inline” with 2023, after previously guiding for growth. Atlas Copco manufactures industrial equipme n Within IT, the strategy benefited from being overweight semi c Lasertec Infineon rt Infineon specializes in automotive semiconductors and the recent pullback is related to decelerating auto demand, albeit from unprecedentedly high levels. TOP PERFORMANCE CONTRIBUTORS Partially offsett in Taiwan Semiconductor Manufacturing Company |
| Performance Past Does Not Indicate Future [Text] | Keep in mind that the Fund’s past performance is not a good predictor of how the Fund will perform in the future. |
| Line Graph [Table Text Block] | |
| Average Annual Return [Table Text Block] | Average Annual Total Returns 1 Year 5 Years 10 Years International Leaders Fund Class N -0.76 % 2.10 % 5.85 % MSCI All Country World ex-U.S. IMI (net) 5.23 % 4.12 % 4.91 % |
| No Deduction of Taxes [Text Block] | The graph and table do not reflect the deduction of taxes that a shareholder would pay on Fund distributions or redemption of Fund shares. |
| Updated Performance Information Location [Text Block] | Visit www.williamblairfunds.com/funds/total-returns |
| Net Assets | $ 1,160,666,000 |
| Holdings Count | Holding | 66 |
| Advisory Fees Paid, Amount | $ 8,632,000 |
| Investment Company, Portfolio Turnover | 66% |
| Additional Fund Statistics [Text Block] | Key Fund S ta Fund net assets (in $000s) $ Number of portfolio holdings 66 Net advisory fees paid (in $000s) $ Portfolio turnover rate 66 % |
| Holdings [Text Block] | Graphical Representation of Holdings The tables below show the investment makeup of the Fund Top Ten Holdings Taiwan Semiconductor Manufacturing Co. Ltd. 4.3 % SAP SE 2.6 % MediaTek, Inc. 2.3 % Schneider Electric SE 2.2 % Industria de Diseno Textil SA 2.2 % London Stock Exchange Group PLC 2.1 % Constellation Software, Inc. 2.1 % Compass Group PLC 2.0 % Sandoz Group AG 2.0 % Intact Financial Corp. 2.0 % Sector Allocation Geogra p |
| Largest Holdings [Text Block] | Top Ten Holdings Taiwan Semiconductor Manufacturing Co. Ltd. 4.3 % SAP SE 2.6 % MediaTek, Inc. 2.3 % Schneider Electric SE 2.2 % Industria de Diseno Textil SA 2.2 % London Stock Exchange Group PLC 2.1 % Constellation Software, Inc. 2.1 % Compass Group PLC 2.0 % Sandoz Group AG 2.0 % Intact Financial Corp. 2.0 % |
| William Blair International Leaders Fund Class R6 | |
| Shareholder Report [Line Items] | |
| Fund Name | William Blair InternationalLeaders Fund |
| Class Name | Class R6 |
| Trading Symbol | WILJX |
| Annual or Semi-Annual Statement [Text Block] | This annual shareholder report contains important information about William Blair International Leaders Fund Class R6 (the "Fund”) for the period of January 1, 2024, to December 31, 2024. |
| Shareholder Report Annual or Semi-Annual | annual shareholder report |
| Additional Information [Text Block] | You can find additional information about the Fund at https://www.williamblairfunds.com/literature/forms |
| Additional Information Phone Number | 1-800-742-7272 |
| Additional Information Website | https://www.williamblairfunds.com/literature/forms |
| Expenses [Text Block] | What were the Fund costs for the last year? (Based on a hypothetical $10,000 investment) Fund Cost of $10,000 Investment Cost of $10,000 Investment as a percentage International Leaders Fund Class R6 $ 0.85 % |
| Expenses Paid, Amount | $ 85 |
| Expense Ratio, Percent | 0.85% |
| Factors Affecting Performance [Text Block] | Management’s Discussion of Fund Performance SUMMARY OF RESULTS The Fund returned -0.51% (net of fees) for the 12 months ended December 31, 2024. The Fund underperformed its benchmark, the MSCI All Country World ex-U.S. IMI (net), which returned 5.23%. The Fund's benchmark is also a broad measure of market performance. Underperformance versus the benchmark was driven by stock selection on both a sector and regional view. From a sector perspective, allocation effect was positive due to underweight exposures to Materials and Consumer Staples and overweight exposures to Information Technology (“IT”) and Industrials. This was partially offset by an underweight to Financials. Selection effect was negative primarily within Industrials, IT, and Financials. On a regional view, allocation effect was negative due to an underweight to China and overweight in Europe. Selection effect was negative primarily in Europe and Japan, though this was partially offset by positive selection in Latin America. Selection within Industrials was also negative and driven in part by positions in Vinci Kingspan Atlas Copco Kingspan has developed a differentiated technology within the building materials industry that may continue to gain market share globally given the strong value proposition it offers customers. The stock underperformed in the fourth quarter after forward guidance called for the company’s full year 2024 to be “broadly inline” with 2023, after previously guiding for growth. Atlas Copco manufactures industrial equipment for industrial use such as air compressors, industrial vacuums, pumps, generators, and power tools. Recent results have been unfavorable given softness in construction and sluggish industrial activity. Within IT, the strategy benefited from being overweight semiconductors, but selection within semiconductors was negative due in part to positions in Lasertec Infineon Infineon specializes in automotive semiconductors and the recent pullback is related to decelerating auto demand, albeit from unprecedentedly high levels. TOP PERFORMANCE CONTRIBUTORS Partially offsetting this was strong performance from Taiwan Semiconductor Manufacturing Company |
| Performance Past Does Not Indicate Future [Text] | Keep in mind that the Fund’s past performance is not a good predictor of how the Fund will perform in the future. |
| Line Graph [Table Text Block] | |
| Average Annual Return [Table Text Block] | Average Annual Total Returns 1 Year 5 Years 10 Years International Leaders Fund Class R6 -0.51 % 2.40 % 6.18 % MSCI All Country World ex-U.S. IMI (net) 5.23 % 4.12 % 4.91 % |
| No Deduction of Taxes [Text Block] | The graph and table do not reflect the deduction of taxes that a shareholder would pay on Fund distributions or redemption of Fund shares. |
| Updated Performance Information Location [Text Block] | Visit www.williamblairfunds.com/funds/total-returns |
| Net Assets | $ 1,160,666,000 |
| Holdings Count | Holding | 66 |
| Advisory Fees Paid, Amount | $ 8,632,000 |
| Investment Company, Portfolio Turnover | 66% |
| Additional Fund Statistics [Text Block] | Key Fund Statistics Fund net assets (in $000s) $ Number of portfolio holdings 66 Net advisory fees paid (in $000s) $ Portfolio turnover rate 66 % |
| Holdings [Text Block] | Graphical Representation of Holdings The tables below show the investment makeup of the Fund, representing percentage of the total net Top Ten Holdings Taiwan Semiconductor Manufacturing Co. Ltd. 4.3 % SAP SE 2.6 % MediaTek, Inc. 2.3 % Schneider Electric SE 2.2 % Industria de Diseno Textil SA 2.2 % London Stock Exchange Group PLC 2.1 % Constellation Software, Inc. 2.1 % Compass Group PLC 2.0 % Sandoz Group AG 2.0 % Intact Financial Corp. 2.0 % Sector Allocation Geographic Diversification |
| Largest Holdings [Text Block] | Top Ten Holdings Taiwan Semiconductor Manufacturing Co. Ltd. 4.3 % SAP SE 2.6 % MediaTek, Inc. 2.3 % Schneider Electric SE 2.2 % Industria de Diseno Textil SA 2.2 % London Stock Exchange Group PLC 2.1 % Constellation Software, Inc. 2.1 % Compass Group PLC 2.0 % Sandoz Group AG 2.0 % Intact Financial Corp. 2.0 % |
| William Blair International Growth Fund - Class I | |
| Shareholder Report [Line Items] | |
| Fund Name | William Blair InternationalGrowth Fund |
| Class Name | Class I |
| Trading Symbol | BIGIX |
| Annual or Semi-Annual Statement [Text Block] | This annual shareholder report contains important information about William Blair International Growth Fund Class I (the "Fund”) for the period of January 1, 2024, to December 31, 2024. |
| Shareholder Report Annual or Semi-Annual | annual shareholder report |
| Additional Information [Text Block] | You can find additional information about the Fund at https://www.williamblairfunds.com/literature/forms |
| Additional Information Phone Number | 1-800-742-7272 |
| Additional Information Website | https://www.williamblairfunds.com/literature/forms |
| Expenses [Text Block] | What were the Fund costs for the last year? (Based on a hypothetical $10,000 investment) Fund Cost of $10,000 Investment Cost of $10,000 Investment as a percentage International Growth Fund Class I $ 0.99 % |
| Expenses Paid, Amount | $ 100 |
| Expense Ratio, Percent | 0.99% |
| Factors Affecting Performance [Text Block] | Management’s Discussion of Fund Performance SUMMARY OF RESULTS The Fund returned 2.38% (net of fees) for the 12 months ended December 31, 2024. The Fund underperformed its benchmark, the MSCI All Country World ex-U.S. IMI (net), which returned 5.23%. The Fund's benchmark is also a broad measure of market performance. Underperformance versus the benchmark TOP PERFORMANCE DETRACTORS Within Industrials, negative selection came primarily from Ryanair SMC Corp Spirax Group SMC Corp is solely focused on the factory automation market and is a dominant global player in the pneumatic control segment. The cyclical recovery in industrial production has been pushed out later than initially anticipated, but datapoints confirm that capex has inflected, albeit at a slower pace than previous cycles. Spirax Group is the global leader in steam applications including the steam trap, which separates steam from water, as well as boiler controls, pressure controls, flow meters, temperature controls and safety valves. The stock has underperformed given the delay in the cyclical recovery of industrial production. Selection within IT was negative for the year primarily driven by Lasertec Dassault Systemes Dassault is a software company specializing in manufacturing and engineering design, with clients in aerospace and defense, transportation, industrial equipment and automation, and healthcare and life sciences. The stock has underperformed this year as revenue and earnings guidance by the company has been recalibrated lower. TOP PERFORMANCE CONTRIBUTORS Selection in Consumer Discretionary was positive led by Dollarama |
| Performance Past Does Not Indicate Future [Text] | Keep in mind that the Fund’s past performance is not a good predictor of how the Fund will perform in the future. |
| Line Graph [Table Text Block] | |
| Average Annual Return [Table Text Block] | Average Annual Total Returns 1 Year 5 Years 10 Years International Growth Fund Class I 2.38 % 4.03 % 5.15 % MSCI All Country World ex-U.S. IMI (net) 5.23 % 4.12 % 4.91 % |
| No Deduction of Taxes [Text Block] | The graph and table do not reflect the deduction of taxes that a shareholder would pay on Fund distributions or redemption of Fund shares. |
| Updated Performance Information Location [Text Block] | Visit www.williamblairfunds.com/funds/total-returns |
| Net Assets | $ 1,363,132,000 |
| Holdings Count | Holding | 187 |
| Advisory Fees Paid, Amount | $ 12,152,000 |
| Investment Company, Portfolio Turnover | 48% |
| Additional Fund Statistics [Text Block] | Key Fund Statist ics Fund net assets (in $000s) $ Number of portfolio holdings 187 Net advisory fees paid (in $000s) $ Portfolio turnover rate 48 % |
| Holdings [Text Block] | Graphical Representation of Holdings The tables below show the investment makeup of the Fund, representing percentage of the total net assets of the Fund. Top Ten Holdings Taiwan Semiconductor Manufacturing Co. Ltd. 3.9 % Tencent Holdings Ltd. 2.0 % SAP SE 1.7 % 3i Group PLC 1.6 % London Stock Exchange Group PLC 1.3 % Hermes International SCA 1.3 % Lonza Group AG 1.2 % DSV AS 1.2 % Bank Central Asia Tbk. PT 1.2 % MTU Aero Engines AG 1.2 % Sector Allocation Geographic Diversification |
| Largest Holdings [Text Block] | Top Ten Holdings Taiwan Semiconductor Manufacturing Co. Ltd. 3.9 % Tencent Holdings Ltd. 2.0 % SAP SE 1.7 % 3i Group PLC 1.6 % London Stock Exchange Group PLC 1.3 % Hermes International SCA 1.3 % Lonza Group AG 1.2 % DSV AS 1.2 % Bank Central Asia Tbk. PT 1.2 % MTU Aero Engines AG 1.2 % |
| William Blair International Growth Fund - Class N | |
| Shareholder Report [Line Items] | |
| Fund Name | William Blair InternationalGrowth Fund |
| Class Name | Class N |
| Trading Symbol | WBIGX |
| Annual or Semi-Annual Statement [Text Block] | This annual shareholder report contains important information about William Blair International Growth Fund Class N (the "Fund”) for the period of January 1, 2024, to December 31, 2024. |
| Shareholder Report Annual or Semi-Annual | annual shareholder report |
| Additional Information [Text Block] | You can find additional information about the Fund at https://www.williamblairfunds.com/literature/forms |
| Additional Information Phone Number | 1-800-742-7272 |
| Additional Information Website | https://www.williamblairfunds.com/literature/forms |
| Expenses [Text Block] | What were the Fund costs for the last year? (Based on a hypothetical $10,000 investment) Fund Cost of $10,000 Investment Cost of $10,000 Investment as a percentage International Growth Fund Class N $ 1.24 % |
| Expenses Paid, Amount | $ 125 |
| Expense Ratio, Percent | 1.24% |
| Factors Affecting Performance [Text Block] | Management’s Discussion of Fund Performance SUMMARY OF RESULTS The Fund returned 2.11% (net of fees) for the 12 months ended December 31, 2024. The Fund underperformed its benchmark, the MSCI All Country World ex-U.S. IMI (net), which returned 5.23%. The Fund's benchmark is also a broad measure of market performance. Underperformance versus the benchmark TOP PERFORMANCE DETRACTORS Within Industrials, negative selection came primarily from Ryanair SMC Corp Spirax Group SMC Corp is solely focused on the factory automation market and is a dominant global player in the pneumatic control segment. The cyclical recovery in industrial production has been pushed out later than initially anticipated, but datapoints confirm that capex has inflected, albeit at a slower pace than previous cycles. Spirax Group is the global leader in steam applications including the steam trap, which separates steam from water, as well as boiler controls, pressure controls, flow meters, temperature controls and safety valves. The stock has underperformed given the delay in the cyclical recovery of industrial production. Selection within IT was negative for the year primarily driven by Lasertec Dassault Systemes Dassault is a software company specializing in manufacturing and engineering design, with clients in aerospace and defense, transportation, industrial equipment and automation, and healthcare and life sciences. The stock has underperformed this year as revenue and earnings guidance by the company has been recalibrated lower. TOP PERFORMANCE CONTRIBUTORS Selection in Consumer Discretionary was positive led by Dollarama |
| Performance Past Does Not Indicate Future [Text] | Keep in mind that the Fund’s past performance is not a good predictor of how the Fund will perform in the future. |
| Line Graph [Table Text Block] | |
| Average Annual Return [Table Text Block] | Average Annual Total Returns 1 Year 5 Years 10 Years International Growth Fund Class N 2.11 % 3.75 % 4.85 % MSCI All Country World ex-U.S. IMI (net) 5.23 % 4.12 % 4.91 % |
| No Deduction of Taxes [Text Block] | The graph and table do not reflect the deduction of taxes that a shareholder would pay on Fund distributions or redemption of Fund shares. |
| Updated Performance Information Location [Text Block] | Visit www.williamblairfunds.com/funds/total-returns |
| Net Assets | $ 1,363,132,000 |
| Holdings Count | Holding | 187 |
| Advisory Fees Paid, Amount | $ 12,152,000 |
| Investment Company, Portfolio Turnover | 48% |
| Additional Fund Statistics [Text Block] | Key Fund Statistics Fund net assets (in $000s) $ Number of portfolio holdings 187 Net advisory fees paid (in $000s) $ Portfolio turnover rate 48 % |
| Holdings [Text Block] | Graphical Representation of Holdings The tables below show the investment makeup of the Fund, representing percentage of the total net assets of the Fund. Top Ten Holdings Taiwan Semiconductor Manufacturing Co. Ltd. 3.9 % Tencent Holdings Ltd. 2.0 % SAP SE 1.7 % 3i Group PLC 1.6 % London Stock Exchange Group PLC 1.3 % Hermes International SCA 1.3 % Lonza Group AG 1.2 % DSV AS 1.2 % Bank Central Asia Tbk. PT 1.2 % MTU Aero Engines AG 1.2 % Sector Allocation Geographic Diversification |
| Largest Holdings [Text Block] | Top Ten Holdings Taiwan Semiconductor Manufacturing Co. Ltd. 3.9 % Tencent Holdings Ltd. 2.0 % SAP SE 1.7 % 3i Group PLC 1.6 % London Stock Exchange Group PLC 1.3 % Hermes International SCA 1.3 % Lonza Group AG 1.2 % DSV AS 1.2 % Bank Central Asia Tbk. PT 1.2 % MTU Aero Engines AG 1.2 % |
| William Blair International Growth Fund - Class R6 | |
| Shareholder Report [Line Items] | |
| Fund Name | William Blair InternationalGrowth Fund |
| Class Name | Class R6 |
| Trading Symbol | WBIRX |
| Annual or Semi-Annual Statement [Text Block] | This annual shareholder report contains important information about William Blair International Growth Fund Class R6 (the "Fund”) for the period of January 1, 2024, to December 31, 2024. |
| Shareholder Report Annual or Semi-Annual | annual shareholder report |
| Additional Information [Text Block] | You can find additional information about the Fund at https://www.williamblairfunds.com/literature/forms |
| Additional Information Phone Number | 1-800-742-7272 |
| Additional Information Website | https://www.williamblairfunds.com/literature/forms |
| Expenses [Text Block] | What were the Fund costs for the last year? (Based on a hypothetical $10,000 investment) Fund Cost of $10,000 Investment Cost of $10,000 Investment as a percentage International Growth Fund Class R6 $ 0.94 % |
| Expenses Paid, Amount | $ 95 |
| Expense Ratio, Percent | 0.94% |
| Factors Affecting Performance [Text Block] | Management’s Discussion of Fund Performance SUMMARY OF RESULTS The Fund returned 2.43% (net of fees) for the 12 months ended December 31, 2024. The Fund underperformed its benchmark, the MSCI All Country World ex-U.S. IMI (net), which returned 5.23%. The Fund's benchmark is also a broad measure of market performance. Underperformance versus the benchmark TOP PERFORMANCE DETRACTORS Within Industrials, negative selection came primarily from Ryanair SMC Corp Spirax Group SMC Corp is solely focused on the factory automation market and is a dominant global player in the pneumatic control segment. The cyclical recovery in industrial production has been pushed out later than initially anticipated, but datapoints confirm that capex has inflected, albeit at a slower pace than previous cycles. Spirax Group is the global leader in steam applications including the steam trap, which separates steam from water, as well as boiler controls, pressure controls, flow meters, temperature controls and safety valves. The stock has underperformed given the delay in the cyclical recovery of industrial production. Selection within IT was negative for the year primarily driven by Lasertec Dassault Systemes Dassault is a software company specializing in manufacturing and engineering design, with clients in aerospace and defense, transportation, industrial equipment and automation, and healthcare and life sciences. The stock has underperformed this year as revenue and earnings guidance by the company has been recalibrated lower. TOP PERFORMANCE CONTRIBUTORS Selection in Consumer Discretionary was positive led by Dollarama |
| Performance Past Does Not Indicate Future [Text] | Keep in mind that the Fund’s past performance is not a good predictor of how the Fund will perform in the future. |
| Line Graph [Table Text Block] | |
| Average Annual Return [Table Text Block] | Average Annual Total Returns 1 Year 5 Years Since Inception International Growth Fund Class R6 2.43 % 4.10 % 5.65 % MSCI All Country World ex-U.S. IMI (net) 5.23 % 4.12 % 4.93 % |
| No Deduction of Taxes [Text Block] | The graph and table do not reflect the deduction of taxes that a shareholder would pay on Fund distributions or redemption of Fund shares. |
| Updated Performance Information Location [Text Block] | Visit www.williamblairfunds.com/funds/total-returns |
| Net Assets | $ 1,363,132,000 |
| Holdings Count | Holding | 187 |
| Advisory Fees Paid, Amount | $ 12,152,000 |
| Investment Company, Portfolio Turnover | 48% |
| Additional Fund Statistics [Text Block] | Key Fund Statistics Fund net assets (in $000s) $ Number of portfolio holdings 187 Net advisory fees paid (in $000s) $ Portfolio turnover rate 48 % |
| Holdings [Text Block] | Graphical Representation of Holdings The tables below show the investment makeup of the Fund, representing percentage of the total net assets of the Fund. Top Ten Holdings Taiwan Semiconductor Manufacturing Co. Ltd. 3.9 % Tencent Holdings Ltd. 2.0 % SAP SE 1.7 % 3i Group PLC 1.6 % London Stock Exchange Group PLC 1.3 % Hermes International SCA 1.3 % Lonza Group AG 1.2 % DSV AS 1.2 % Bank Central Asia Tbk. PT 1.2 % MTU Aero Engines AG 1.2 % Sector Allocation Geographic Diversification |
| Largest Holdings [Text Block] | Top Ten Holdings Taiwan Semiconductor Manufacturing Co. Ltd. 3.9 % Tencent Holdings Ltd. 2.0 % SAP SE 1.7 % 3i Group PLC 1.6 % London Stock Exchange Group PLC 1.3 % Hermes International SCA 1.3 % Lonza Group AG 1.2 % DSV AS 1.2 % Bank Central Asia Tbk. PT 1.2 % MTU Aero Engines AG 1.2 % |
| William Blair Institutional International Growth Fund - Institutional Class | |
| Shareholder Report [Line Items] | |
| Fund Name | William Blair InstitutionalInternational Growth Fund |
| Class Name | Institutional Class |
| Trading Symbol | WBIIX |
| Annual or Semi-Annual Statement [Text Block] | This annual shareholder report contains important information about William Blair Institutional International Growth Fund Institutional Class (the "Fund”) for the period of January 1, 2024, to December 31, 2024. |
| Shareholder Report Annual or Semi-Annual | annual shareholder report |
| Additional Information [Text Block] | You can find additional information about the Fund at https://www.williamblairfunds.com/literature/forms |
| Additional Information Phone Number | 1-800-742-7272 |
| Additional Information Website | https://www.williamblairfunds.com/literature/forms |
| Expenses [Text Block] | What were the Fund costs for the last year? (Based on a hypothetical $10,000 investment) Fund Cost of $10,000 Investment Cost of $10,000 Investment as a percentage Institutional International Growth Fund $ 0.94 % |
| Expenses Paid, Amount | $ 95 |
| Expense Ratio, Percent | 0.94% |
| Factors Affecting Performance [Text Block] | Management’s Discussion of Fund Performance SUMMARY OF RESULTS The Fund returned 2.41% (net of fees) for the 12 months ended December 31, 2024. The Fund underperformed its benchmark, the MSCI All Country World ex-U.S. IMI (net), which returned 5.23%. The Fund's benchmark is also a broad measure of market performance. Underperformance versus the benchmark TOP PERFORMANCE DETRACTORS Within Industrials, negative selection came primarily from Ryanair SMC Corp Spirax Group on SMC Corp is solely focused on the factory automation market and is a dominant global player in the pneumatic control segment. The cyclical recovery in industrial production has been pushed out later than initially anticipated, but datapoints confirm that capex has inflected, albeit at a slower pace than previous cycles. Spirax Group is the global leader in steam applications including the steam trap, which separates steam from water, as well as boiler controls, pressure controls, flow meters, temperature controls and safety valves. The stock has underperformed given the delay in the cyclical recovery of industrial production. Selection within IT was negative for the year primarily driven by Lasertec Dassault Systemes Dassault is a software company specializing in manufacturing and engineering design, with clients in aerospace and defense, transportation, industrial equipment and automation, and healthcare and life sciences. The stock has underperformed this year as revenue and earnings guidance by the company has been recalibrated lower. TOP PERFORMANCE CONTRIBUTORS Selection in Consumer Discretionary was positive led by Dollarama |
| Performance Past Does Not Indicate Future [Text] | Keep in mind that the Fund’s past performance is not a good predictor of how the Fund will perform in the future. |
| Line Graph [Table Text Block] | |
| Average Annual Return [Table Text Block] | Average Annual Total Returns 1 Year 5 Years 10 Years Institutional International Growth Fund 2.41 % 4.16 % 5.28 % MSCI All Country World ex-U.S. IMI (net) 5.23 % 4.12 % 4.91 % |
| No Deduction of Taxes [Text Block] | The graph and table do not reflect the deduction of taxes that a shareholder would pay on Fund distributions or redemption of Fund shares. |
| Updated Performance Information Location [Text Block] | Visit www.williamblairfunds.com/funds/total-returns |
| Net Assets | $ 897,044,000 |
| Holdings Count | Holding | 187 |
| Advisory Fees Paid, Amount | $ 8,484,000 |
| Investment Company, Portfolio Turnover | 50% |
| Additional Fund Statistics [Text Block] | Key Fund Statistics Fund net assets (in $000s) $ Number of portfolio holdings 187 Net advisory fees paid (in $000s) $ Portfolio turnover rate 50 % |
| Holdings [Text Block] | Graphical Representation of Holdings The tables below show the investment makeup of the Fund, representing percentage Top Ten Holdings Taiwan Semiconductor Manufacturing Co. Ltd. 3.9 % Tencent Holdings Ltd. 2.1 % SAP SE 1.7 % 3i Group PLC 1.6 % London Stock Exchange Group PLC 1.3 % Hermes International SCA 1.3 % Lonza Group AG 1.2 % DSV AS 1.2 % Bank Central Asia Tbk. PT 1.2 % MTU Aero Engines AG 1.2 % Sector Allocation Geographic Diversification |
| Largest Holdings [Text Block] | Top Ten Holdings Taiwan Semiconductor Manufacturing Co. Ltd. 3.9 % Tencent Holdings Ltd. 2.1 % SAP SE 1.7 % 3i Group PLC 1.6 % London Stock Exchange Group PLC 1.3 % Hermes International SCA 1.3 % Lonza Group AG 1.2 % DSV AS 1.2 % Bank Central Asia Tbk. PT 1.2 % MTU Aero Engines AG 1.2 % |
| William Blair International Small Cap Growth Fund Class I | |
| Shareholder Report [Line Items] | |
| Fund Name | William Blair InternationalSmall Cap Growth Fund |
| Class Name | Class I |
| Trading Symbol | WISIX |
| Annual or Semi-Annual Statement [Text Block] | This annual shareholder report contains important information about William Blair International Small Cap Growth Fund Class I (the "Fund”) for the period of January 1, 2024, to December 31, 2024. |
| Shareholder Report Annual or Semi-Annual | annual shareholder report |
| Additional Information [Text Block] | You can find additional information about the Fund at https://www.williamblairfunds.com/literature/forms |
| Additional Information Phone Number | 1-800-742-7272 |
| Additional Information Website | https://www.williamblairfunds.com/literature/forms |
| Expenses [Text Block] | What were the Fund costs for the last year? (Based on a hypothetical $10,000 investment) Fund Cost of $10,000 Investment Cost of $10,000 Investment as a percentage International Small Cap Growth Fund Class I $ 1.10 % |
| Expenses Paid, Amount | $ 110 |
| Expense Ratio, Percent | 1.10% |
| Factors Affecting Performance [Text Block] | Management’s Discussion of Fund Performance SUMMARY OF RESULTS The Fund returned 0.80% (net of fees) for the 12 months ended December 31, 2024. The Fund underperformed its benchmark, the MSCI All Country World ex-U.S. Small Cap Index (net), which returned 3.36% and underperformed the MSCI All Country World ex-U.S. IMI (net), a broad measure of market performance, which returned 5.23%. Underperformance has been driven by weaker stock selection within Japanese Industrials and Information Technology (“IT”) sectors, and an overweight to Mexico. Japan positioning and stock selection has hampered relative performance, particularly our exposure to domestic-oriented, higher-value, human-capital-intensive business models. These companies have underperformed more recently by wage inflation and a significant weakening in the yen. TOP PERFORMANCE DETRACTORS Within Industrials, SMS Co Ltd. TechnoPro IT’s weaker stock selection was primarily due to Shift Additionally in IT, Siltronic TOP PERFORMANCE CONTRIBUTORS IT underperformance was partially offset by a position in Technology One Also offsetting underperformance was positive stock selection within Developed Asia and Europe as well as the Health Care and Communication Services sectors. Relative performance in Health Care was anchored by Pro Medicus Communication services positive stock selection was driven by positions in Trustpilot Group CTS Eventim |
| Performance Past Does Not Indicate Future [Text] | Keep in mind that the Fund’s past performance is not a good predictor of how the Fund will perform in the future. |
| Line Graph [Table Text Block] | |
| Average Annual Return [Table Text Block] | Average Annual Total Returns 1 Year 5 Years 10 Years International Small Cap Growth Fund Class I 0.80 % 1.52 % 4.33 % MSCI All Country World ex-U.S. IMI (net) 5.23 % 4.12 % 4.91 % MSCI All Country World ex-U.S. Small Cap Index (net) 3.36 % 4.30 % 5.66 % |
| No Deduction of Taxes [Text Block] | The graph and table do not reflect the deduction of taxes that a shareholder would pay on Fund distributions or redemption of Fund shares. |
| Updated Performance Information Location [Text Block] | Visit www.williamblairfunds.com/funds/total-returns |
| Net Assets | $ 160,700,000 |
| Holdings Count | Holding | 114 |
| Advisory Fees Paid, Amount | $ 1,746,000 |
| Investment Company, Portfolio Turnover | 49% |
| Additional Fund Statistics [Text Block] | Key Fund Statistics Fund net assets (in $000s) $ Number of portfolio holdings 114 Net advisory fees paid (in $000s) $ Portfolio turnover rate 49 % |
| Holdings [Text Block] | Graphical Representation of Holdings The tables below show the investment makeup of the Fund, representing percentage of the total net assets of the Fund. Top Ten Holdings Pro Medicus Ltd. 3.3 % Technology One Ltd. 2.6 % Rakuten Bank Ltd. 2.0 % Diploma PLC 2.0 % Trustpilot Group PLC 2.0 % North West Co., Inc. 1.8 % Lifco AB 1.7 % Beazley PLC 1.7 % Lotes Co. Ltd. 1.6 % BayCurrent Consulting, Inc. 1.6 % Sector Allocation Geographic Diversification |
| Largest Holdings [Text Block] | Top Ten Holdings Pro Medicus Ltd. 3.3 % Technology One Ltd. 2.6 % Rakuten Bank Ltd. 2.0 % Diploma PLC 2.0 % Trustpilot Group PLC 2.0 % North West Co., Inc. 1.8 % Lifco AB 1.7 % Beazley PLC 1.7 % Lotes Co. Ltd. 1.6 % BayCurrent Consulting, Inc. 1.6 % |
| William Blair International Small Cap Growth Fund - Class N | |
| Shareholder Report [Line Items] | |
| Fund Name | William Blair InternationalSmall Cap Growth Fund |
| Class Name | Class N |
| Trading Symbol | WISNX |
| Annual or Semi-Annual Statement [Text Block] | This annual shareholder report contains important information about William Blair International Small Cap Growth Fund Class N (the "Fund”) for the period of January 1, 2024, to December 31, 2024. |
| Shareholder Report Annual or Semi-Annual | annual shareholder report |
| Additional Information [Text Block] | You can find additional information about the Fund at https://www.williamblairfunds.com/literature/forms |
| Additional Information Phone Number | 1-800-742-7272 |
| Additional Information Website | https://www.williamblairfunds.com/literature/forms |
| Expenses [Text Block] | What were the Fund costs for the last year? (Based on a hypothetical $10,000 investment) Fund Cost of $10,000 Investment Cost of $10,000 Investment as a percentage International Small Cap Growth Fund Class N $ 1.35 % |
| Expenses Paid, Amount | $ 135 |
| Expense Ratio, Percent | 1.35% |
| Factors Affecting Performance [Text Block] | Management’s Discussion of Fund Performance SUMMARY OF RESULTS The Fund returned 0.55% (net of fees) for the 12 months ended December 31, 2024. The Fund underperformed its benchmark, the MSCI All Country World ex-U.S. Small Cap Index (net), which returned 3.36% and underperformed the MSCI All Country World ex-U.S. IMI (net), a broad measure of market performance, which returned 5.23%. Underperformance has been driven by weaker stock selection within Japanese Industrials and Information TOP PERFORMANCE DETRACTORS Within Industrials, SMS Co Ltd. TechnoPro IT’s weaker stock selection was primarily due to Shift Additionally in IT, Siltronic TOP PERFORMANCE CONTRIBUTORS IT underperformance was partially offset by a position in Technology One Also offsetting underperformance was positive stock selection within Developed Asia and Europe as well as the Health Care and Communication Services sectors. Relative performance in Health Care was anchored by Pro Medicus Communication services positive stock selection was driven by positions in Trustpilot Group CTS Eventim |
| Performance Past Does Not Indicate Future [Text] | Keep in mind that the Fund’s past performance is not a good predictor of how the Fund will perform in the future. |
| Line Graph [Table Text Block] | |
| Average Annual Return [Table Text Block] | Average Annual Total Returns 1 Year 5 Years 10 Years International Small Cap Growth Fund Class N 0.55 % 1.26 % 4.05 % MSCI All Country World ex-U.S. IMI (net) 5.23 % 4.12 % 4.91 % MSCI All Country World ex-U.S. Small Cap Index (net) 3.36 % 4.30 % 5.66 % |
| No Deduction of Taxes [Text Block] | The graph and table do not reflect the deduction of taxes that a shareholder would pay on Fund distributions or redemption of Fund shares. |
| Updated Performance Information Location [Text Block] | Visit www.williamblairfunds.com/funds/total-returns |
| Net Assets | $ 160,700,000 |
| Holdings Count | Holding | 114 |
| Advisory Fees Paid, Amount | $ 1,746,000 |
| Investment Company, Portfolio Turnover | 49% |
| Additional Fund Statistics [Text Block] | Key Fund Statistics Fund net assets (in $000s) $ Number of portfolio holdings 114 Net advisory fees paid (in $000s) $ Portfolio turnover rate 49 % |
| Holdings [Text Block] | Graphical Representation of Holdings The tables below show the investment makeup of the Fund, representing percentage of the total net assets of the Fund. Top Ten Holdings Pro Medicus Ltd. 3.3 % Technology One Ltd. 2.6 % Rakuten Bank Ltd. 2.0 % Diploma PLC 2.0 % Trustpilot Group PLC 2.0 % North West Co., Inc. 1.8 % Lifco AB 1.7 % Beazley PLC 1.7 % Lotes Co. Ltd. 1.6 % BayCurrent Consulting, Inc. 1.6 % Sector Allocation Geographic Diversification |
| Largest Holdings [Text Block] | Top Ten Holdings Pro Medicus Ltd. 3.3 % Technology One Ltd. 2.6 % Rakuten Bank Ltd. 2.0 % Diploma PLC 2.0 % Trustpilot Group PLC 2.0 % North West Co., Inc. 1.8 % Lifco AB 1.7 % Beazley PLC 1.7 % Lotes Co. Ltd. 1.6 % BayCurrent Consulting, Inc. 1.6 % |
| William Blair International Small Cap Growth Fund - Class R6 | |
| Shareholder Report [Line Items] | |
| Fund Name | William Blair InternationalSmall Cap Growth Fund |
| Class Name | Class R6 |
| Trading Symbol | WIISX |
| Annual or Semi-Annual Statement [Text Block] | This annual shareholder report contains important information about William Blair International Small Cap Growth Fund Class R6 (the "Fund”) for the period of January 1, 2024, to December 31, 2024. |
| Shareholder Report Annual or Semi-Annual | annual shareholder report |
| Additional Information [Text Block] | You can find additional information about the Fund at https://www.williamblairfunds.com/literature/forms |
| Additional Information Phone Number | 1-800-742-7272 |
| Additional Information Website | https://www.williamblairfunds.com/literature/forms |
| Expenses [Text Block] | What were the Fund costs for the last year? (Based on a hypothetical $10,000 investment) Fund Cost of $10,000 Investment Cost of $10,000 Investment as a percentage International Small Cap Growth Fund Class R6 $ 1.05 % |
| Expenses Paid, Amount | $ 105 |
| Expense Ratio, Percent | 1.05% |
| Factors Affecting Performance [Text Block] | Management’s Discussion of Fund Performance SUMMARY OF RESULTS The Fund returned 0.86% (net of fees) for the 12 months ended December Underperformance has been driven by weaker stock selection within Japanese Industrials and Information Technology (“IT”) sectors, and an overweight to Mexico. Japan positioning and stock selection has hampered relative performance, particularly our exposure to domestic-oriented, higher-value, human-capital-intensive business models. These companies have underperformed more recently by wage inflation and a significant weakening in the yen. TOP PERFORMANCE DETRACTORS Within Industrials, SMS Co Ltd. TechnoPro IT’s weaker stock selection was primarily due to Shift Additionally in IT, Siltronic TOP PERFORMANCE CONTRIBUTORS IT underperformance was partially offset by a position in Technology One Also offsetting underperformance was positive stock selection within Developed Asia and Europe as well as the Health Care and Communication Services sectors. Relative performance in Health Care was anchored by Pro Medicus Communication services positive stock selection was driven by positions in Trustpilot Group CTS Eventim |
| Performance Past Does Not Indicate Future [Text] | Keep in mind that the Fund’s past performance is not a good predictor of how the Fund will perform in the future. |
| Line Graph [Table Text Block] | |
| Average Annual Return [Table Text Block] | Average Annual Total Returns 1 Year 5 Years 10 Years International Small Cap Growth Fund Class R6 0.86 % 1.60 % 4.42 % MSCI All Country World ex-U.S. IMI (net) 5.23 % 4.12 % 4.91 % MSCI All Country World ex-U.S. Small Cap Index (net) 3.36 % 4.30 % 5.66 % |
| No Deduction of Taxes [Text Block] | The graph and table do not reflect the deduction of taxes that a shareholder would pay on Fund distributions or redemption of Fund shares. |
| Updated Performance Information Location [Text Block] | Visit www.williamblairfunds.com/funds/total-returns |
| Net Assets | $ 160,700,000 |
| Holdings Count | Holding | 114 |
| Advisory Fees Paid, Amount | $ 1,746,000 |
| Investment Company, Portfolio Turnover | 49% |
| Additional Fund Statistics [Text Block] | Key Fund Statistics Fund net assets (in $000s) $ Number of portfolio holdings 114 Net advisory fees paid (in $000s) $ Portfolio turnover rate 49 % |
| Holdings [Text Block] | Graphical Representation of Holdings The tables below show the investment makeup of the Fund, representing percentage of the total net assets of the Fund. Top Ten Holdings Pro Medicus Ltd. 3.3 % Technology One Ltd. 2.6 % Rakuten Bank Ltd. 2.0 % Diploma PLC 2.0 % Trustpilot Group PLC 2.0 % North West Co., Inc. 1.8 % Lifco AB 1.7 % Beazley PLC 1.7 % Lotes Co. Ltd. 1.6 % BayCurrent Consulting, Inc. 1.6 % Sector Allocation Geographic Diversification |
| Largest Holdings [Text Block] | Top Ten Holdings Pro Medicus Ltd. 3.3 % Technology One Ltd. 2.6 % Rakuten Bank Ltd. 2.0 % Diploma PLC 2.0 % Trustpilot Group PLC 2.0 % North West Co., Inc. 1.8 % Lifco AB 1.7 % Beazley PLC 1.7 % Lotes Co. Ltd. 1.6 % BayCurrent Consulting, Inc. 1.6 % |
| William Blair Emerging Markets Leaders Fund - Class I | |
| Shareholder Report [Line Items] | |
| Fund Name | William Blair Emerging MarketsLeaders Fund |
| Class Name | Class I |
| Trading Symbol | WBELX |
| Annual or Semi-Annual Statement [Text Block] | This annual shareholder report contains important information about William Blair Emerging Markets Leaders Fund Class I (the "Fund”) for the period of January 1, 2024, to December 31, 2024. |
| Shareholder Report Annual or Semi-Annual | annual shareholder report |
| Additional Information [Text Block] | You can find additional information about the Fund at https://www.williamblairfunds.com/literature/forms |
| Additional Information Phone Number | 1-800-742-7272 |
| Additional Information Website | https://www.williamblairfunds.com/literature/forms |
| Expenses [Text Block] | What were the Fund costs for the last year? (Based on a hypothetical $10,000 investment) Fund Cost of $10,000 Investment Cost of $10,000 Investment as a percentage Emerging Markets Leaders Fund Class I $ 0.99 % |
| Expenses Paid, Amount | $ 102 |
| Expense Ratio, Percent | 0.99% |
| Factors Affecting Performance [Text Block] | Management’s Discussion of Fund Performance SUMMARY OF RESULTS The Fund returned 5.86% (net of fees) for the 12 months ended December 31, 2024. The Fund underperformed The slight underperformance against the benchmark during the year was driven by a combination of an overweight to Latin America and stock selection within the Financials, Information Technology (“IT”), and Consumer Staples sectors. Within Financials, BTG Pactual and HDFC Bank were notable detractors to relative results, while Samsung, Raia Drogasil, and WalMart de Mexico were meaningful detractors in the IT and Consumer Staples sectors. TOP PERFORMANCE DETRACTORS HDFC Bank BTG Pactual Within IT, strong performance from the semiconductor space including a meaningful position in Taiwan Semiconductor Manufacturing Company Samsung Technology The overweight to Latin America, including positions in Raia Drogasil WalMart de Mexico TOP PERFORMANCE CONTRIBUTORS Partly offsetting these effects was positive sector allocation led by an overweight to IT and specifically semiconductors, and an underweight to Materials, as well as stock selection within the Consumer Discretionary and Industrials sectors. Within Consumer Discretionary, Chinese positions initiated in 2024 including Trip.com Meituan Positive stock selection in Industrials was driven by positions in Contemporary Amperex Technology Company Sungrow Power Supply |
| Performance Past Does Not Indicate Future [Text] | Keep in mind that the Fund’s past performance is not a good predictor of how the Fund will perform in the future. |
| Line Graph [Table Text Block] | |
| Average Annual Return [Table Text Block] | Average Annual Total Returns 1 Year 5 Years 10 Years Emerging Markets Leaders Fund Class I 5.86 % -0.34 % 2.57 % MSCI Emerging Markets Index (net) 7.50 % 1.70 % 3.64 % |
| No Deduction of Taxes [Text Block] | The graph and table do not reflect the deduction of taxes that a shareholder would pay on Fund distributions or redemption of Fund shares. |
| Updated Performance Information Location [Text Block] | Visit www.williamblairfunds.com/funds/total-returns |
| Net Assets | $ 164,701,000 |
| Holdings Count | Holding | 64 |
| Advisory Fees Paid, Amount | $ 1,633,000 |
| Investment Company, Portfolio Turnover | 67% |
| Additional Fund Statistics [Text Block] | Key Fund Statistics Fund net assets (in $000s) $ Number of portfolio holdings 64 Net advisory fees paid (in $000s) $ Portfolio turnover rate 67 % |
| Holdings [Text Block] | Graphical Representation of Holdings The tables below show the investment makeup of the Fund, representing percentage of the total net assets of the Fund. Top Ten Holdings Taiwan Semiconductor Manufacturing Co. Ltd. 10.9 % Tencent Holdings Ltd. 6.5 % Trip.com Group Ltd. 4.0 % MediaTek, Inc. 3.0 % Kotak Mahindra Bank Ltd. 2.5 % Meituan 2.5 % Bank Central Asia Tbk. PT 2.3 % SK Hynix, Inc. 2.2 % Contemporary Amperex Technology Co. Ltd. 2.1 % Delta Electronics, Inc. 2.0 % Sector Allocation Geographic Diversification |
| Largest Holdings [Text Block] | Top Ten Holdings Taiwan Semiconductor Manufacturing Co. Ltd. 10.9 % Tencent Holdings Ltd. 6.5 % Trip.com Group Ltd. 4.0 % MediaTek, Inc. 3.0 % Kotak Mahindra Bank Ltd. 2.5 % Meituan 2.5 % Bank Central Asia Tbk. PT 2.3 % SK Hynix, Inc. 2.2 % Contemporary Amperex Technology Co. Ltd. 2.1 % Delta Electronics, Inc. 2.0 % |
| William Blair Emerging Markets Leaders Fund - Class N | |
| Shareholder Report [Line Items] | |
| Fund Name | William Blair Emerging MarketsLeaders Fund |
| Class Name | Class N |
| Trading Symbol | WELNX |
| Annual or Semi-Annual Statement [Text Block] | This annual shareholder report contains important information about William Blair Emerging Markets Leaders Fund Class N (the "Fund”) for the period of January 1, 2024, to December 31, 2024. |
| Shareholder Report Annual or Semi-Annual | annual shareholder report |
| Additional Information [Text Block] | You can find additional information about the Fund at https://www.williamblairfunds.com/literature/forms |
| Additional Information Phone Number | 1-800-742-7272 |
| Additional Information Website | https://www.williamblairfunds.com/literature/forms |
| Expenses [Text Block] | What were the Fund costs for the last year? (Based on a hypothetical $10,000 investment) Fund Cost of $10,000 Investment Cost of $10,000 Investment as a percentage Emerging Markets Leaders Fund Class N $ 1.24 % |
| Expenses Paid, Amount | $ 128 |
| Expense Ratio, Percent | 1.24% |
| Factors Affecting Performance [Text Block] | Management’s Discussion of Fund Performance SUMMARY OF RESULTS The Fund returned 5.69% (net of fees) for the 12 months ended December 31, 2024. The Fund The slight underperformance against the benchmark during the year was driven by a combination of an overweight to Latin America and stock selection within the Financials, Information Technology (“IT”), and Consumer Staples sectors. Within Financials, BTG Pactual and HDFC Bank were notable detractors to relative results, while Samsung, Raia Drogasil, and WalMart de Mexico were meaningful detractors in the IT and Consumer Staples sectors. TOP PERFORMANCE DETRACTORS HDFC Bank BTG Pactual Within IT, strong performance from the semiconductor space including a meaningful position in Taiwan Semiconductor Manufacturing Company Samsung Technology The overweight to Latin America, including positions in Raia Drogasil WalMart de Mexico TOP PERFORMANCE CONTRIBUTORS Partly offsetting these effects was positive sector allocation led by an overweight to IT and specifically semiconductors, and an underweight to Materials, as well as stock selection within the Consumer Discretionary and Industrials sectors. Within Consumer Discretionary, Chinese positions initiated in 2024 including Trip.com Meituan Positive stock selection in Industrials was driven by positions in Contemporary Amperex Technology Company Sungrow Power Supply |
| Performance Past Does Not Indicate Future [Text] | Keep in mind that the Fund’s past performance is not a good predictor of how the Fund will perform in the future. |
| Line Graph [Table Text Block] | |
| Average Annual Return [Table Text Block] | Average Annual Total Returns 1 Year 5 Years 10 Years Emerging Markets Leaders Fund Class N 5.69 % -0.59 % 2.30 % MSCI Emerging Markets Index (net) 7.50 % 1.70 % 3.64 % |
| No Deduction of Taxes [Text Block] | The graph and table do not reflect the deduction of taxes that a shareholder would pay on Fund distributions or redemption of Fund shares. |
| Updated Performance Information Location [Text Block] | Visit www.williamblairfunds.com/funds/total-returns |
| Net Assets | $ 164,701,000 |
| Holdings Count | Holding | 64 |
| Advisory Fees Paid, Amount | $ 1,633,000 |
| Investment Company, Portfolio Turnover | 67% |
| Additional Fund Statistics [Text Block] | Key Fund Statistics Fund net assets (in $000s) $ Number of portfolio holdings 64 Net advisory fees paid (in $000s) $ Portfolio turnover rate 67 % |
| Holdings [Text Block] | Graphical Representation of Holdings The tables below show the investment makeup of the Fund, representing percentage of the total net assets of the Fund. Top Ten Holdings Taiwan Semiconductor Manufacturing Co. Ltd. 10.9 % Tencent Holdings Ltd. 6.5 % Trip.com Group Ltd. 4.0 % MediaTek, Inc. 3.0 % Kotak Mahindra Bank Ltd. 2.5 % Meituan 2.5 % Bank Central Asia Tbk. PT 2.3 % SK Hynix, Inc. 2.2 % Contemporary Amperex Technology Co. Ltd. 2.1 % Delta Electronics, Inc. 2.0 % Sector Allocation Geographic Diversification |
| Largest Holdings [Text Block] | Top Ten Holdings Taiwan Semiconductor Manufacturing Co. Ltd. 10.9 % Tencent Holdings Ltd. 6.5 % Trip.com Group Ltd. 4.0 % MediaTek, Inc. 3.0 % Kotak Mahindra Bank Ltd. 2.5 % Meituan 2.5 % Bank Central Asia Tbk. PT 2.3 % SK Hynix, Inc. 2.2 % Contemporary Amperex Technology Co. Ltd. 2.1 % Delta Electronics, Inc. 2.0 % |
| William Blair Emerging Markets Leaders Fund - Class R6 | |
| Shareholder Report [Line Items] | |
| Fund Name | William Blair Emerging MarketsLeaders Fund |
| Class Name | Class R6 |
| Trading Symbol | WELIX |
| Annual or Semi-Annual Statement [Text Block] | This annual shareholder report contains important information about William Blair Emerging Markets Leaders Fund Class R6 (the "Fund”) for the period of January 1, 2024, to December 31, 2024. |
| Shareholder Report Annual or Semi-Annual | annual shareholder report |
| Additional Information [Text Block] | You can find additional information about the Fund at https://www.williamblairfunds.com/literature/forms |
| Additional Information Phone Number | 1-800-742-7272 |
| Additional Information Website | https://www.williamblairfunds.com/literature/forms |
| Expenses [Text Block] | What were the Fund costs for the last year? (Based on a hypothetical $10,000 investment) Fund Cost of $10,000 Investment Cost of $10,000 Investment as a percentage Emerging Markets Leaders Fund Class R6 $ 0.94 % |
| Expenses Paid, Amount | $ 97 |
| Expense Ratio, Percent | 0.94% |
| Factors Affecting Performance [Text Block] | Management’s Discussion of Fund Performance SUMMARY OF RESULTS The Fund returned 5.92% (net of fees) for the 12 months ended December 31, 2024. The Fund underperformed The slight underperformance against the benchmark during the year was driven by a combination of an overweight to Latin America and stock selection within the Financials, Information Technology (“IT”), and Consumer Staples sectors. Within Financials, BTG Pactual and HDFC Bank were notable detractors to relative results, while Samsung, Raia Drogasil, and WalMart de Mexico were meaningful detractors in the IT and Consumer Staples sectors. TOP PERFORMANCE DETRACTORS HDFC Bank BTG Pactual Within IT, strong performance from the semiconductor space including a meaningful position in Taiwan Semiconductor Manufacturing Company Samsung Technology The overweight to Latin America, including positions in Raia Drogasil WalMart de Mexico TOP PERFORMANCE CONTRIBUTORS Partly offsetting these effects was positive sector allocation led by an overweight to IT and specifically semiconductors, and an underweight to Materials, as well as stock selection within the Consumer Discretionary and Industrials sectors. Within Consumer Discretionary, Chinese positions initiated in 2024 including Trip.com Meituan Positive stock selection in Industrials was driven by positions in Contemporary Amperex Technology Company Sungrow Power Supply |
| Performance Past Does Not Indicate Future [Text] | Keep in mind that the Fund’s past performance is not a good predictor of how the Fund will perform in the future. |
| Line Graph [Table Text Block] | |
| Average Annual Return [Table Text Block] | Average Annual Total Returns 1 Year 5 Years 10 Years Emerging Markets Leaders Fund Class R6 5.92 % -0.30 % 2.62 % MSCI Emerging Markets Index (net) 7.50 % 1.70 % 3.64 % |
| No Deduction of Taxes [Text Block] | The graph and table do not reflect the deduction of taxes that a shareholder would pay on Fund distributions or redemption of Fund shares. |
| Updated Performance Information Location [Text Block] | Visit www.williamblairfunds.com/funds/total-returns |
| Net Assets | $ 164,701,000 |
| Holdings Count | Holding | 64 |
| Advisory Fees Paid, Amount | $ 1,633,000 |
| Investment Company, Portfolio Turnover | 67% |
| Additional Fund Statistics [Text Block] | Key Fund Statistics Fund net assets (in $000s) $ Number of portfolio holdings 64 Net advisory fees paid (in $000s) $ Portfolio turnover rate 67 % |
| Holdings [Text Block] | Graphical Representation of Holdings The tables below show the investment make percentage Top Ten Holdings Taiwan Semiconductor Manufacturing Co. Ltd. 10.9 % Tencent Holdings Ltd. 6.5 % Trip.com Group Ltd. 4.0 % MediaTek, Inc. 3.0 % Kotak Mahindra Bank Ltd. 2.5 % Meituan 2.5 % Bank Central Asia Tbk. PT 2.3 % SK Hynix, Inc. 2.2 % Contemporary Amperex Technology Co. Ltd. 2.1 % Delta Electronics, Inc. 2.0 % Sector Allocation Geographic Diversification |
| Largest Holdings [Text Block] | Top Ten Holdings Taiwan Semiconductor Manufacturing Co. Ltd. 10.9 % Tencent Holdings Ltd. 6.5 % Trip.com Group Ltd. 4.0 % MediaTek, Inc. 3.0 % Kotak Mahindra Bank Ltd. 2.5 % Meituan 2.5 % Bank Central Asia Tbk. PT 2.3 % SK Hynix, Inc. 2.2 % Contemporary Amperex Technology Co. Ltd. 2.1 % Delta Electronics, Inc. 2.0 % |
| William Blair Emerging Markets Growth Fund - Class I | |
| Shareholder Report [Line Items] | |
| Fund Name | William Blair Emerging MarketsGrowth Fund |
| Class Name | Class I |
| Trading Symbol | WBEIX |
| Annual or Semi-Annual Statement [Text Block] | This annual shareholder report contains important information about William Blair Emerging Markets Growth Fund Class I (the "Fund”) for the period of January 1, 2024, to December 31, 2024. |
| Shareholder Report Annual or Semi-Annual | annual shareholder report |
| Additional Information [Text Block] | You can find additional information about the Fund at https://www.williamblairfunds.com/literature/forms |
| Additional Information Phone Number | 1-800-742-7272 |
| Additional Information Website | https://www.williamblairfunds.com/literature/forms |
| Expenses [Text Block] | What were the Fund costs for the last year? (Based on a hypothetical $10,000 investment) Fund Cost of $10,000 Investment Cost of $10,000 Investment as a percentage Emerging Markets Growth Fund Class I $ 0.99 % |
| Expenses Paid, Amount | $ 104 |
| Expense Ratio, Percent | 0.99% |
| Factors Affecting Performance [Text Block] | Management’s Discussion of Fund Performance SUMMARY OF RESULTS The Fund returned 10.62% (net of fees) for the 12 months ended December 31, 2024. The Fund outperformed its benchmark, the MSCI Emerging Markets IMI (net), which returned 7.09%. The Fund's benchmark is also a broad measure of market performance. TOP PERFORMANCE CONTRIBUTORS Within Consumer Discretionary, Dixon Technologies India Trent HD Hyundai Electric Co Ltd TOP PERFORMANCE DETRACTORS Partly offsetting these positive effects was negative stock selection within Financials, dragged down by AIA Group Bank Rakyat Indonesia Persero Tbk PT HDFC Bank |
| Performance Past Does Not Indicate Future [Text] | Keep in mind that the Fund’s past performance is not a good predictor of how the Fund will perform in the future. |
| Line Graph [Table Text Block] | |
| Average Annual Return [Table Text Block] | Average Annual Total Returns 1 Year 5 Years 10 Years Emerging Markets Growth Fund Class I 10.62 % 3.44 % 4.42 % MSCI Emerging Markets IMI (net) 7.09 % 2.51 % 3.90 % |
| No Deduction of Taxes [Text Block] | The graph and table do not reflect the deduction of taxes that a shareholder would pay on Fund distributions or redemption of Fund shares. |
| Updated Performance Information Location [Text Block] | Visit www.williamblairfunds.com/funds/total-returns |
| Net Assets | $ 880,034,000 |
| Holdings Count | Holding | 132 |
| Advisory Fees Paid, Amount | $ 6,602,000 |
| Investment Company, Portfolio Turnover | 81% |
| Additional Fund Statistics [Text Block] | Key Fund Statistics Fund net assets (in $000s) $ Number of portfolio holdings 132 Net advisory fees paid (in $000s) $ Portfolio turnover rate 81 % |
| Holdings [Text Block] | Graphical Representation of Holdings The tables below show the investment makeup of the Fund, representing percentage of the total net assets of the Fund. Top Ten Holdings Taiwan Semiconductor Manufacturing Co. Ltd. 13.3 % Tencent Holdings Ltd. 6.7 % Trip.com Group Ltd. 3.6 % Meituan 3.3 % SK Hynix, Inc. 2.5 % Bank Central Asia Tbk. PT 2.5 % MercadoLibre, Inc. 2.3 % Sea Ltd. 1.8 % Xiaomi Corporation 1.7 % ICICI Bank Ltd. 1.5 % Sector Allocation Geographic Diversification |
| Largest Holdings [Text Block] | Top Ten Holdings Taiwan Semiconductor Manufacturing Co. Ltd. 13.3 % Tencent Holdings Ltd. 6.7 % Trip.com Group Ltd. 3.6 % Meituan 3.3 % SK Hynix, Inc. 2.5 % Bank Central Asia Tbk. PT 2.5 % MercadoLibre, Inc. 2.3 % Sea Ltd. 1.8 % Xiaomi Corporation 1.7 % ICICI Bank Ltd. 1.5 % |
| William Blair Emerging Markets Growth Fund - Class N | |
| Shareholder Report [Line Items] | |
| Fund Name | William Blair Emerging MarketsGrowth Fund |
| Class Name | Class N |
| Trading Symbol | WBENX |
| Annual or Semi-Annual Statement [Text Block] | This annual shareholder report contains important information about William Blair Emerging Markets Growth Fund Class N (the "Fund”) for the period of January 1, 2024, to December 31, 2024. |
| Shareholder Report Annual or Semi-Annual | annual shareholder report |
| Additional Information [Text Block] | You can find additional information about the Fund at https://www.williamblairfunds.com/literature/forms |
| Additional Information Phone Number | 1-800-742-7272 |
| Additional Information Website | https://www.williamblairfunds.com/literature/forms |
| Expenses [Text Block] | What were the Fund costs for the last year? (Based on a hypothetical $10,000 investment) Fund Cost of $10,000 Investment Cost of $10,000 Investment as a percentage Emerging Markets Growth Fund Class N $ 1.24 % |
| Expenses Paid, Amount | $ 130 |
| Expense Ratio, Percent | 1.24% |
| Factors Affecting Performance [Text Block] | Management’s Discussion of Fund Performance SUMMARY OF RESULTS The Fund returned 10.40% (net of fees) for the 12 months ended December 31, 2024. The Fund outperformed its benchmark, the MSCI Emerging Markets IMI (net), which returned 7.09%. The Fund's benchmark is also a broad measure of market performance. Outperformance versus the benchmark allocation TOP PERFORMANCE CONTRIBUTORS Within Consumer Discretionary, Dixon Technologies India Trent HD Hyundai Electric Co Ltd TOP PERFORMANCE DETRACTORS Partly offsetting these positive effects was negative stock selection within Financials, dragged down by AIA Group Bank Rakyat Indonesia Persero Tbk PT HDFC Bank |
| Performance Past Does Not Indicate Future [Text] | Keep in mind that the Fund’s past performance is not a good predictor of how the Fund will perform in the future. |
| Line Graph [Table Text Block] | |
| Average Annual Return [Table Text Block] | Average Annual Total Returns 1 Year 5 Years 10 Years Emerging Markets Growth Fund Class N 10.40 % 3.20 % 4.16 % MSCI Emerging Markets IMI (net) 7.09 % 2.51 % 3.90 % |
| No Deduction of Taxes [Text Block] | The graph and table do not reflect the deduction of taxes that a shareholder would pay on Fund distributions or redemption of Fund shares. |
| Updated Performance Information Location [Text Block] | Visit www.williamblairfunds.com/funds/total-returns |
| Net Assets | $ 880,034,000 |
| Holdings Count | Holding | 132 |
| Advisory Fees Paid, Amount | $ 6,602,000 |
| Investment Company, Portfolio Turnover | 81% |
| Additional Fund Statistics [Text Block] | Key Fund Statistics Fund net assets (in $000s) $ Number of portfolio holdings 132 Net advisory fees paid (in $000s) $ Portfolio turnover rate 81 % |
| Holdings [Text Block] | Graphical Representation of Holdings The tables below show the investment makeup of the Fund, representing percentage of the total net assets of the Fund. Top Ten Holdings Taiwan Semiconductor Manufacturing Co. Ltd. 13.3 % Tencent Holdings Ltd. 6.7 % Trip.com Group Ltd. 3.6 % Meituan 3.3 % SK Hynix, Inc. 2.5 % Bank Central Asia Tbk. PT 2.5 % MercadoLibre, Inc. 2.3 % Sea Ltd. 1.8 % Xiaomi Corporation 1.7 % ICICI Bank Ltd. 1.5 % Sector Allocation Geographic Diversification |
| Largest Holdings [Text Block] | Top Ten Holdings Taiwan Semiconductor Manufacturing Co. Ltd. 13.3 % Tencent Holdings Ltd. 6.7 % Trip.com Group Ltd. 3.6 % Meituan 3.3 % SK Hynix, Inc. 2.5 % Bank Central Asia Tbk. PT 2.5 % MercadoLibre, Inc. 2.3 % Sea Ltd. 1.8 % Xiaomi Corporation 1.7 % ICICI Bank Ltd. 1.5 % |
| William Blair Emerging Markets Growth Fund - Class R6 | |
| Shareholder Report [Line Items] | |
| Fund Name | William Blair Emerging MarketsGrowth Fund |
| Class Name | Class R6 |
| Trading Symbol | BIEMX |
| Annual or Semi-Annual Statement [Text Block] | This annual shareholder report contains important information about William Blair Emerging Markets Growth Fund Class R6 (the "Fund”) for the period of January 1, 2024, to December 31, 2024. |
| Shareholder Report Annual or Semi-Annual | annual shareholder report |
| Additional Information [Text Block] | You can find additional information about the Fund at https://www.williamblairfunds.com/literature/forms |
| Additional Information Phone Number | 1-800-742-7272 |
| Additional Information Website | https://www.williamblairfunds.com/literature/forms |
| Expenses [Text Block] | What were the Fund costs for the last year? (Based on a hypothetical $10,000 investment) Fund Cost of $10,000 Investment Cost of $10,000 Investment as a percentage Emerging Markets Growth Fund Class R6 $ 0.94 % |
| Expenses Paid, Amount | $ 99 |
| Expense Ratio, Percent | 0.94% |
| Factors Affecting Performance [Text Block] | Management’s Discussion of Fund Performance SUMMARY OF RESULTS The Fund returned 10.71% (net of fees) for the 12 months ended December 31, 2024. The Fund outperformed its benchmark, the MSCI Emerging Markets IMI (net), which returned 7.09%. The Fund's benchmark is also a broad measure of market performance. Outperformance versus the benchmark TOP PERFORMANCE CONTRIBUTORS Within Consumer Discretionary, Dixon Technologies India Trent robust HD Hyundai Electric Co Ltd er tra TOP PERFORMANCE DETRACTORS Partly offsetting these positive effects was negative stock selection within Financials, dragged down by AIA Group Bank Rakyat Indonesia Persero Tbk PT HDFC Bank |
| Performance Past Does Not Indicate Future [Text] | Keep in mind that the Fund’s past performance is not a good predictor of how the Fund will perform in the future. |
| Line Graph [Table Text Block] | |
| Average Annual Return [Table Text Block] | Average Annual Total Returns 1 Year 5 Years 10 Years Emerging Markets Growth Fund Class R6 10.71 % 3.52 % 4.50 % MSCI Emerging Markets IMI (net) 7.09 % 2.51 % 3.90 % |
| No Deduction of Taxes [Text Block] | The graph and table do not reflect the deduction of taxes that a shareholder would pay on Fund distributions or redemption of Fund shares. |
| Updated Performance Information Location [Text Block] | Visit www.williamblairfunds.com/funds/total-returns |
| Net Assets | $ 880,034,000 |
| Holdings Count | Holding | 132 |
| Advisory Fees Paid, Amount | $ 6,602,000 |
| Investment Company, Portfolio Turnover | 81% |
| Additional Fund Statistics [Text Block] | Key Fund Statistics Fund net assets (in $000s) $ Number of portfolio holdings 132 Net advisory fees paid (in $000s) $ Portfolio turnover rate 81 % |
| Holdings [Text Block] | Graphical Representation of Holdings The tables below show the investment makeup of the Fund, representing percentage t asset the Fu Top Ten Holdings Taiwan Semiconductor Manufacturing Co. Ltd. 13.3 % Tencent Holdings Ltd. 6.7 % Trip.com Group Ltd. 3.6 % Meituan 3.3 % SK Hynix, Inc. 2.5 % Bank Central Asia Tbk. PT 2.5 % MercadoLibre, Inc. 2.3 % Sea Ltd. 1.8 % Xiaomi Corporation 1.7 % ICICI Bank Ltd. 1.5 % Sector Allocation Geographic Diversification |
| Largest Holdings [Text Block] | Top Ten Holdings Taiwan Semiconductor Manufacturing Co. Ltd. 13.3 % Tencent Holdings Ltd. 6.7 % Trip.com Group Ltd. 3.6 % Meituan 3.3 % SK Hynix, Inc. 2.5 % Bank Central Asia Tbk. PT 2.5 % MercadoLibre, Inc. 2.3 % Sea Ltd. 1.8 % Xiaomi Corporation 1.7 % ICICI Bank Ltd. 1.5 % |
| William Blair Emerging Markets ex China Growth Fund - Class 1 | |
| Shareholder Report [Line Items] | |
| Fund Name | William Blair Emerging Marketsex China Growth Fund |
| Class Name | Class I |
| Trading Symbol | WXCIX |
| Annual or Semi-Annual Statement [Text Block] | This annual shareholder report contains important information about William Blair Emerging Markets ex China Growth Fund Class I (the "Fund”) for the period of January 1, 2024, to December 31, 2024. |
| Shareholder Report Annual or Semi-Annual | annual shareholder report |
| Additional Information [Text Block] | You can find additional information about the Fund at https://www.williamblairfunds.com/literature/forms |
| Additional Information Phone Number | 1-800-742-7272 |
| Additional Information Website | https://www.williamblairfunds.com/literature/forms |
| Expenses [Text Block] | What were the Fund costs for the last year? (Based on a hypothetical $10,000 investment) Fund Cost of $10,000 Investment Cost of $10,000 Investment as a percentage Emerging Markets ex China Growth Fund Class I $ 0.99 % |
| Expenses Paid, Amount | $ 106 |
| Expense Ratio, Percent | 0.99% |
| Factors Affecting Performance [Text Block] | Management’s Discussion of Fund Performance SUMMARY OF RESULTS The Fund returned 13.49% (net of fees) for the 12 months ended December 31, 2024. The Fund outperformed its benchmark, the MSCI Emerging Markets ex-China IMI (net), which returned 3.70%. The Fund's benchmark is also a broad measure of market performance. Outperformance versus the benchmark year-to-date was driven by a combination of stock selection and allocation effects. Stock selection was particularly strong within the Industrials, Consumer Discretionary, and Information Technology (“IT”) sectors. Furthermore, the Materials sector TOP PERFORMANCE CONTRIBUTORS Industrials performance was bolstered by Hd Hyundai Electric Co Ltd Dixon Technologies India Trent Taiwan Semiconductor Manufacturing Company TOP PERFORMANCE DETRACTORS Partially offsetting these positive effects was negative stock selection within Financials coupled with Mexico, Brazil, and Indonesia overweighting. HDFC Bank Bank Rakyat Indonesia |
| Performance Past Does Not Indicate Future [Text] | Keep in mind that the Fund’s past performance is not a good predictor of how the Fund will perform in the future. |
| Line Graph [Table Text Block] | |
| Average Annual Return [Table Text Block] | Average Annual Total Returns 1 Year Since Inception Emerging Markets ex China Growth Fund Class I 13.49 % 12.53 % MSCI Emerging Markets ex-China IMI (net) 3.70 % 9.39 % |
| No Deduction of Taxes [Text Block] | The graph and table do not reflect the deduction of taxes that a shareholder would pay on Fund distributions or redemption of Fund shares. |
| Updated Performance Information Location [Text Block] | Visit www.williamblairfunds.com/funds/total-returns |
| Net Assets | $ 35,472,000 |
| Holdings Count | Holding | 118 |
| Advisory Fees Paid, Amount | $ 0 |
| Investment Company, Portfolio Turnover | 69% |
| Additional Fund Statistics [Text Block] | Key Fund Statistics Fund net assets (in $000s) $ Number of portfolio holdings 118 Net advisory fees paid (in $000s) $ Portfolio turnover rate 69 % |
| Holdings [Text Block] | Graphical Representation of Holdings The tables below show the investment makeup of the Fund, representing percentage of the total net assets of the Fund. Top Ten Holdings Taiwan Semiconductor Manufacturing Co. Ltd. 16.5 % SK Hynix, Inc. 2.5 % MediaTek, Inc. 1.9 % MercadoLibre, Inc. 1.9 % Bank Central Asia Tbk. PT 1.8 % Sea Ltd. 1.6 % Emaar Properties PJSC 1.5 % Dixon Technologies India Ltd. 1.5 % Trent Ltd. 1.4 % Indian Hotels Co. Ltd. 1.4 % Sector Allocation Geographic Diversification |
| Largest Holdings [Text Block] | Top Ten Holdings Taiwan Semiconductor Manufacturing Co. Ltd. 16.5 % SK Hynix, Inc. 2.5 % MediaTek, Inc. 1.9 % MercadoLibre, Inc. 1.9 % Bank Central Asia Tbk. PT 1.8 % Sea Ltd. 1.6 % Emaar Properties PJSC 1.5 % Dixon Technologies India Ltd. 1.5 % Trent Ltd. 1.4 % Indian Hotels Co. Ltd. 1.4 % |
| William Blair Emerging Markets ex China Growth Fund - Class R6 | |
| Shareholder Report [Line Items] | |
| Fund Name | William Blair Emerging Marketsex China Growth Fund |
| Class Name | Class R6 |
| Trading Symbol | WXCRX |
| Annual or Semi-Annual Statement [Text Block] | This annual shareholder report contains important information about William Blair Emerging Markets ex China Growth Fund Class R6 (the "Fund”) for the period of January 1, 2024, to December 31, 2024. |
| Shareholder Report Annual or Semi-Annual | annual shareholder report |
| Additional Information [Text Block] | You can find additional information about the Fund at https://www.williamblairfunds.com/literature/forms |
| Additional Information Phone Number | 1-800-742-7272 |
| Additional Information Website | https://www.williamblairfunds.com/literature/forms |
| Expenses [Text Block] | What were the Fund costs for the last year? (Based on a hypothetical $10,000 investment) Fund Cost of $10,000 Investment Cost of $10,000 Investment as a percentage Emerging Markets ex China Growth Fund Class R6 $ 0.94 % |
| Expenses Paid, Amount | $ 100 |
| Expense Ratio, Percent | 0.94% |
| Factors Affecting Performance [Text Block] | Management’s Discussion of Fund Performance SUMMARY OF RESULTS The Fund returned 13.56% (net of fees) for the 12 months ended December 31, 2024. The Fund outperformed its benchmark, the MSCI Emerging Markets ex-China IMI (net), which returned 3.70%. The Fund's benchmark is also a broad measure of market performance. Outperformance versus the benchmark year-to-date was driven by a combination of stock selection and allocation effects. Stock selection was particularly strong within the Industrials, Consumer Discretionary, and Information Technology (“IT”) sectors. Furthermore, the Materials sector TOP PERFORMANCE CONTRIBUTORS Industrials performance was bolstered by Hd Hyundai Electric Co Ltd Dixon Technologies India Trent Taiwan Semiconductor Manufacturing Company TOP PERFORMANCE DETRACTORS Partially offsetting these positive effects was negative stock selection within Financials coupled with Mexico, Brazil, and Indonesia overweighting. HDFC Bank Bank Rakyat Indonesia |
| Performance Past Does Not Indicate Future [Text] | Keep in mind that the Fund’s past performance is not a good predictor of how the Fund will perform in the future. |
| Line Graph [Table Text Block] | |
| Average Annual Return [Table Text Block] | Average Annual Total Returns 1 Year Since Inception Emerging Markets ex China Growth Fund Class R6 13.56 % 12.61 % MSCI Emerging Markets ex-China IMI (net) 3.70 % 9.39 % |
| No Deduction of Taxes [Text Block] | The graph and table do not reflect the deduction of taxes that a shareholder would pay on Fund distributions or redemption of Fund shares. |
| Updated Performance Information Location [Text Block] | Visit www.williamblairfunds.com/funds/total-returns |
| Net Assets | $ 35,472,000 |
| Holdings Count | Holding | 118 |
| Advisory Fees Paid, Amount | $ 0 |
| Investment Company, Portfolio Turnover | 69% |
| Additional Fund Statistics [Text Block] | Key Fund Statistics Fund net assets (in $000s) $ Number of portfolio holdings 118 Net advisory fees paid (in $000s) $ Portfolio turnover rate 69 % |
| Holdings [Text Block] | Graphical Representation of Holdings The tables below show the investment makeup of the Fund, representing percentage of the total net assets of the Fund. Top Ten Holdings Taiwan Semiconductor Manufacturing Co. Ltd. 16.5 % SK Hynix, Inc. 2.5 % MediaTek, Inc. 1.9 % MercadoLibre, Inc. 1.9 % Bank Central Asia Tbk. PT 1.8 % Sea Ltd. 1.6 % Emaar Properties PJSC 1.5 % Dixon Technologies India Ltd. 1.5 % Trent Ltd. 1.4 % Indian Hotels Co. Ltd. 1.4 % Sector Allocation Geographic Diversification |
| Largest Holdings [Text Block] | Top Ten Holdings Taiwan Semiconductor Manufacturing Co. Ltd. 16.5 % SK Hynix, Inc. 2.5 % MediaTek, Inc. 1.9 % MercadoLibre, Inc. 1.9 % Bank Central Asia Tbk. PT 1.8 % Sea Ltd. 1.6 % Emaar Properties PJSC 1.5 % Dixon Technologies India Ltd. 1.5 % Trent Ltd. 1.4 % Indian Hotels Co. Ltd. 1.4 % |
| William Blair Emerging Markets Small Cap Growth Fund - Class 1 | |
| Shareholder Report [Line Items] | |
| Fund Name | William Blair Emerging MarketsSmall Cap Growth Fund |
| Class Name | Class I |
| Trading Symbol | BESIX |
| Annual or Semi-Annual Statement [Text Block] | This annual shareholder report contains important information about William Blair Emerging Markets Small Cap Growth Fund Class I (the "Fund”) for the period of January 1, 2024, to December 31, 2024. |
| Shareholder Report Annual or Semi-Annual | annual shareholder report |
| Additional Information [Text Block] | You can find additional information about the Fund at https://www.williamblairfunds.com/literature/forms |
| Additional Information Phone Number | 1-800-742-7272 |
| Additional Information Website | https://www.williamblairfunds.com/literature/forms |
| Expenses [Text Block] | What were the Fund costs for the last year? (Based on a hypothetical $10,000 investment) Fund Cost of $10,000 Investment Cost of $10,000 Investment as a percentage Emerging Markets Small Cap Growth Fund Class I $ 1.15 % |
| Expenses Paid, Amount | $ 120 |
| Expense Ratio, Percent | 1.15% |
| Factors Affecting Performance [Text Block] | Management’s Discussion of Fund Performance SUMMARY OF RESULTS The Fund returned 8.37% (net of fees) for the 12 months ended December 31, 2024. The Fund outperformed its benchmark, the MSCI Emerging Markets Small Cap Index (net), which returned 4.79% and outperformed the MSCI Emerging Markets IMI (net), a broad measure of market performance, which returned 7.09%. Outperformance versus the benchmark selection TOP PERFORMANCE CONTRIBUTORS Within Industrials, electrical equipment companies, in particular HD Hyundai Electric Co Ltd ABB India Dixon Technologies India Pop Mart International Group Banco Macro TOP PERFORMANCE DETRACTORS Partially offsetting these positive effects was negative stock selection within Health Care and Information Technology (“IT”). Within Health Care, the lack of exposure to biotechnology and life science tools industries detracted from relative performance. In addition, Oncoclinicas do Brasil Servicos Medicos GDS Holdings Elite Materials |
| Performance Past Does Not Indicate Future [Text] | Keep in mind that the Fund’s past performance is not a good predictor of how the Fund will perform in the future. |
| Line Graph [Table Text Block] | |
| Average Annual Return [Table Text Block] | Average Annual Total Returns 1 Year 5 Years 10 Years Emerging Markets Small Cap Growth Fund Class I 8.37 % 7.90 % 5.42 % MSCI Emerging Markets IMI (net) 7.09 % 2.51 % 3.90 % MSCI Emerging Markets Small Cap Index (net) 4.79 % 8.56 % 5.73 % |
| No Deduction of Taxes [Text Block] | The graph and table do not reflect the deduction of taxes that a shareholder would pay on Fund distributions or redemption of Fund shares. |
| Updated Performance Information Location [Text Block] | Visit www.williamblairfunds.com/funds/total-returns |
| Net Assets | $ 398,093,000 |
| Holdings Count | Holding | 122 |
| Advisory Fees Paid, Amount | $ 3,439,000 |
| Investment Company, Portfolio Turnover | 188% |
| Additional Fund Statistics [Text Block] | Key Fund Statistics Fund net assets (in $000s) $ Number of portfolio holdings 122 Net advisory fees paid (in $000s) $ Portfolio turnover rate 188 % |
| Holdings [Text Block] | Graphical Representation of Holdings The tables below show the investment makeup of the Fund, representing percentage of the total net assets of the Fund. Top Ten Holdings Banco Macro SA 2.5 % Dixon Technologies India Ltd. 2.3 % Emaar Development PJSC 2.2 % Gamuda Bhd. 1.9 % Eastroc Beverage Group Co. Ltd. 1.8 % Bank Syariah Indonesia Tbk. PT 1.7 % Salik Co. PJSC 1.6 % Kaynes Technology India Ltd. 1.6 % Central Depository Services India Ltd. 1.6 % BBB Foods, Inc. 1.5 % Sector Allocation Geographic Diversification |
| Largest Holdings [Text Block] | Top Ten Holdings Banco Macro SA 2.5 % Dixon Technologies India Ltd. 2.3 % Emaar Development PJSC 2.2 % Gamuda Bhd. 1.9 % Eastroc Beverage Group Co. Ltd. 1.8 % Bank Syariah Indonesia Tbk. PT 1.7 % Salik Co. PJSC 1.6 % Kaynes Technology India Ltd. 1.6 % Central Depository Services India Ltd. 1.6 % BBB Foods, Inc. 1.5 % |
| William Blair Emerging Markets Small Cap Growth Fund - Class N | |
| Shareholder Report [Line Items] | |
| Fund Name | William Blair Emerging MarketsSmall Cap Growth Fund |
| Class Name | Class N |
| Trading Symbol | WESNX |
| Annual or Semi-Annual Statement [Text Block] | This annual shareholder report contains important information about William Blair Emerging Markets Small Cap Growth Fund Class N (the "Fund”) for the period of January 1, 2024, to December 31, 2024. |
| Shareholder Report Annual or Semi-Annual | annual shareholder report |
| Additional Information [Text Block] | You can find additional information about the Fund at https://www.williamblairfunds.com/literature/forms |
| Additional Information Phone Number | 1-800-742-7272 |
| Additional Information Website | https://www.williamblairfunds.com/literature/forms |
| Expenses [Text Block] | What were the Fund costs for the last year? (Based on a hypothetical $10,000 investment) Fund Cost of $10,000 Investment Cost of $10,000 Investment as a percentage Emerging Markets Small Cap Growth Fund Class N $ 1.40 % |
| Expenses Paid, Amount | $ 146 |
| Expense Ratio, Percent | 1.40% |
| Factors Affecting Performance [Text Block] | Management’s Discussion of Fund Performance SUMMARY OF RESULTS The Fund returned 8.09% (net of fees) for the 12 months ended December 31, 2024. The Fund outperformed its benchmark, the MSCI Emerging Markets Small Cap Index (net), which returned 4.79% and outperformed the MSCI Emerging Markets IMI (net), a broad measure of market performance, which returned 7.09%. Outperformance versus the benchmark TOP PERFORMANCE CONTRIBUTORS Within Industrials, electrical equipment companies, in particular HD Hyundai Electric Co Ltd ABB India Dixon Technologies India Pop Mart International Group Banco Macro TOP PERFORMANCE DETRACTORS Partially offsetting these positive effects was negative stock selection within Health Care and Information Technology (“IT”). Within Health Care, the lack of exposure to biotechnology and life science tools industries detracted from relative performance. In addition, Oncoclinicas do Brasil Servicos Medicos GDS Holdings Elite Materials |
| Performance Past Does Not Indicate Future [Text] | Keep in mind that the Fund’s past performance is not a good predictor of how the Fund will perform in the future. |
| Line Graph [Table Text Block] | |
| Average Annual Return [Table Text Block] | Average Annual Total Returns 1 Year 5 Years 10 Years Emerging Markets Small Cap Growth Fund Class N 8.09 % 7.62 % 5.13 % MSCI Emerging Markets IMI (net) 7.09 % 2.51 % 3.90 % MSCI Emerging Markets Small Cap Index (net) 4.79 % 8.56 % 5.73 % |
| No Deduction of Taxes [Text Block] | The graph and table do not reflect the deduction of taxes that a shareholder would pay on Fund distributions or redemption of Fund shares. |
| Updated Performance Information Location [Text Block] | Visit www.williamblairfunds.com/funds/total-returns |
| Net Assets | $ 398,093,000 |
| Holdings Count | Holding | 122 |
| Advisory Fees Paid, Amount | $ 3,439,000 |
| Investment Company, Portfolio Turnover | 188% |
| Additional Fund Statistics [Text Block] | Key Fund Statistics Fund net assets (in $000s) $ Number of portfolio holdings 122 Net advisory fees paid (in $000s) $ Portfolio turnover rate 188 % |
| Holdings [Text Block] | Graphical Representation of Holdings The tables below show the investment makeup of the Fund, representing percentage of the total net assets of the Fund. Top Ten Holdings Banco Macro SA 2.5 % Dixon Technologies India Ltd. 2.3 % Emaar Development PJSC 2.2 % Gamuda Bhd. 1.9 % Eastroc Beverage Group Co. Ltd. 1.8 % Bank Syariah Indonesia Tbk. PT 1.7 % Salik Co. PJSC 1.6 % Kaynes Technology India Ltd. 1.6 % Central Depository Services India Ltd. 1.6 % BBB Foods, Inc. 1.5 % Sector Allocation Geographic Diversification |
| Largest Holdings [Text Block] | Top Ten Holdings Banco Macro SA 2.5 % Dixon Technologies India Ltd. 2.3 % Emaar Development PJSC 2.2 % Gamuda Bhd. 1.9 % Eastroc Beverage Group Co. Ltd. 1.8 % Bank Syariah Indonesia Tbk. PT 1.7 % Salik Co. PJSC 1.6 % Kaynes Technology India Ltd. 1.6 % Central Depository Services India Ltd. 1.6 % BBB Foods, Inc. 1.5 % |
| William Blair Emerging Markets Small Cap Growth Fund - Class R6 | |
| Shareholder Report [Line Items] | |
| Fund Name | William Blair Emerging MarketsSmall Cap Growth Fund |
| Class Name | Class R6 |
| Trading Symbol | WESJX |
| Annual or Semi-Annual Statement [Text Block] | This annual shareholder report contains important information about William Blair Emerging Markets Small Cap Growth Fund Class R6 (the "Fund”) for the period of January 1, 2024, to December 31, 2024. |
| Shareholder Report Annual or Semi-Annual | annual shareholder report |
| Additional Information [Text Block] | You can find additional information about the Fund at https://www.williamblairfunds.com/literature/forms |
| Additional Information Phone Number | 1-800-742-7272 |
| Additional Information Website | https://www.williamblairfunds.com/literature/forms |
| Expenses [Text Block] | What were the Fund costs for the last year? (Based on a hypothetical $10,000 investment) Fund Cost of $10,000 Investment Cost of $10,000 Investment as a percentage Emerging Markets Small Cap Growth Fund Class R6 $ 1.10 % |
| Expenses Paid, Amount | $ 115 |
| Expense Ratio, Percent | 1.10% |
| Factors Affecting Performance [Text Block] | Management’s Discussion of Fund Performance SUMMARY OF RESULTS The Fund returned 8.39% (net of fees) for the 12 months ended December 31, 2024. The Fund outperformed its benchmark, the MSCI Emerging Markets Small Cap Index (net), which returned 4.79% and outperformed the MSCI Emerging Markets IMI (net), a broad measure of market performance, which returned 7.09%. Outperformance versus the benchmark TOP PERFORMANCE CONTRIBUTORS Within Industrials, electrical equipment companies, in particular HD Hyundai Electric Co Ltd ABB India Dixon Technologies India Pop Mart International Group Banco Macro TOP PERFORMANCE DETRACTORS Partially offsetting these positive effects was negative stock selection within Health Care and Information Technology (“IT”). Within Health Care, the lack of exposure to biotechnology and life science tools industries detracted from relative performance. In addition, Oncoclinicas do Brasil Servicos Medicos GDS Holdings Elite Materials |
| Performance Past Does Not Indicate Future [Text] | Keep in mind that the Fund’s past performance is not a good predictor of how the Fund will perform in the future. |
| Line Graph [Table Text Block] | |
| Average Annual Return [Table Text Block] | Average Annual Total Returns 1 Year 5 Years 10 Years Emerging Markets Small Cap Growth Fund Class R6 8.39 % 7.95 % 5.48 % MSCI Emerging Markets IMI (net) 7.09 % 2.51 % 3.90 % MSCI Emerging Markets Small Cap Index (net) 4.79 % 8.56 % 5.73 % |
| No Deduction of Taxes [Text Block] | The graph and table do not reflect the deduction of taxes that a shareholder would pay on Fund distributions or redemption of Fund shares. |
| Updated Performance Information Location [Text Block] | Visit www.williamblairfunds.com/funds/total-returns |
| Net Assets | $ 398,093,000 |
| Holdings Count | Holding | 122 |
| Advisory Fees Paid, Amount | $ 3,439,000 |
| Investment Company, Portfolio Turnover | 188% |
| Additional Fund Statistics [Text Block] | Key Fund Statistics Fund net assets (in $000s) $ Number of portfolio holdings 122 Net advisory fees paid (in $000s) $ Portfolio turnover rate 188 % |
| Holdings [Text Block] | Graphical Representation of Holdings The tables below show the investment makeup of the Fund, representing percentage of the total net assets of the Fund. Top Ten Holdings Banco Macro SA 2.5 % Dixon Technologies India Ltd. 2.3 % Emaar Development PJSC 2.2 % Gamuda Bhd. 1.9 % Eastroc Beverage Group Co. Ltd. 1.8 % Bank Syariah Indonesia Tbk. PT 1.7 % Salik Co. PJSC 1.6 % Kaynes Technology India Ltd. 1.6 % Central Depository Services India Ltd. 1.6 % BBB Foods, Inc. 1.5 % Sector Allocation Geographic Diversification |
| Largest Holdings [Text Block] | Top Ten Holdings Banco Macro SA 2.5 % Dixon Technologies India Ltd. 2.3 % Emaar Development PJSC 2.2 % Gamuda Bhd. 1.9 % Eastroc Beverage Group Co. Ltd. 1.8 % Bank Syariah Indonesia Tbk. PT 1.7 % Salik Co. PJSC 1.6 % Kaynes Technology India Ltd. 1.6 % Central Depository Services India Ltd. 1.6 % BBB Foods, Inc. 1.5 % |
| William Blair China Growth Fund - Class 1 | |
| Shareholder Report [Line Items] | |
| Fund Name | William Blair China Growth Fund |
| Class Name | Class I |
| Trading Symbol | WICGX |
| Annual or Semi-Annual Statement [Text Block] | This annual shareholder report contains important information about William Blair China Growth Fund Class I (the "Fund”) for the period of January 1, 2024, to December 31, 2024. |
| Shareholder Report Annual or Semi-Annual | annual shareholder report |
| Additional Information [Text Block] | You can find additional information about the Fund at https://www.williamblairfunds.com/literature/forms |
| Additional Information Phone Number | 1-800-742-7272 |
| Additional Information Website | https://www.williamblairfunds.com/literature/forms |
| Expenses [Text Block] | What were the Fund costs for the last year? (Based on a hypothetical $10,000 investment) Fund Cost of $10,000 Investment Cost of $10,000 Investment as a percentage China Growth Fund Class I $ 0.99 % |
| Expenses Paid, Amount | $ 104 |
| Expense Ratio, Percent | 0.99% |
| Factors Affecting Performance [Text Block] | Management’s Discussion of Fund Performance SUMMARY OF RESULTS The Fund returned 10.34% (net of fees) for the 12 months ended December 31, 2024. The Fund underperformed its benchmark, the MSCI China All Shares Index (net), which returned 16.38%. The Fund's benchmark is also a broad measure of market performance. Underperformance versus the benchmark TOP PERFORMANCE DETRACTORS Within Financials, AIA Group Within IT, Silergy TOP PERFORMANCE CONTRIBUTORS Partly offsetting the detractors from performance was the portfolio’s sector positioning, led by an underweight to Materials and stock selection within Consumer Staples, led by Eastroc Beverage Group |
| Performance Past Does Not Indicate Future [Text] | Keep in mind that the Fund’s past performance is not a good predictor of how the Fund will perform in the future. |
| Line Graph [Table Text Block] | |
| Average Annual Return [Table Text Block] | Average Annual Total Returns 1 Year Since Inception China Growth Fund Class I 10.34 % -16.82 % MSCI China All Shares Index (net) 16.38 % -7.83 % |
| No Deduction of Taxes [Text Block] | The graph and table do not reflect the deduction of taxes that a shareholder would pay on Fund distributions or redemption of Fund shares. |
| Updated Performance Information Location [Text Block] | Visit www.williamblairfunds.com/funds/total-returns |
| Net Assets | $ 1,728,000 |
| Holdings Count | Holding | 45 |
| Advisory Fees Paid, Amount | $ 0 |
| Investment Company, Portfolio Turnover | 81% |
| Additional Fund Statistics [Text Block] | Key Fund Statistics Fund net assets (in $000s) $ Number of portfolio holdings 45 Net advisory fees paid (in $000s) $ Portfolio turnover rate 81 % |
| Holdings [Text Block] | Graphical Representation of Holdings The tables below show the investment makeup of the Fund, representing percentage of the total net assets of the Fund. Top Ten Holdings Tencent Holdings Ltd. 13.4 % Kweichow Moutai Co. Ltd. 5.0 % Meituan 4.5 % Contemporary Amperex Technology Co. Ltd. 4.5 % Trip.com Group Ltd. 4.2 % Xiaomi Corporation 4.1 % East Money Information Co.,Ltd. 3.3 % BYD Company Limited 3.1 % Eastroc Beverage Group Co. Ltd. 2.9 % Yutong Bus Co. Ltd. 2.8 % Sector Allocation Geographic Diversification |
| Largest Holdings [Text Block] | Top Ten Holdings Tencent Holdings Ltd. 13.4 % Kweichow Moutai Co. Ltd. 5.0 % Meituan 4.5 % Contemporary Amperex Technology Co. Ltd. 4.5 % Trip.com Group Ltd. 4.2 % Xiaomi Corporation 4.1 % East Money Information Co.,Ltd. 3.3 % BYD Company Limited 3.1 % Eastroc Beverage Group Co. Ltd. 2.9 % Yutong Bus Co. Ltd. 2.8 % |
| William Blair China Growth Fund - Class R6 | |
| Shareholder Report [Line Items] | |
| Fund Name | William Blair China Growth Fund |
| Class Name | Class R6 |
| Trading Symbol | WRCGX |
| Annual or Semi-Annual Statement [Text Block] | This annual shareholder report contains important information about William Blair China Growth Fund Class R6 (the "Fund”) for the period of January 1, 2024, to December 31, 2024. |
| Shareholder Report Annual or Semi-Annual | annual shareholder report |
| Additional Information [Text Block] | You can find additional information about the Fund at https://www.williamblairfunds.com/literature/forms |
| Additional Information Phone Number | 1-800-742-7272 |
| Additional Information Website | https://www.williamblairfunds.com/literature/forms |
| Expenses [Text Block] | What were the Fund costs for the last year? (Based on a hypothetical $10,000 investment) Fund Cost of $10,000 Investment Cost of $10,000 Investment as a percentage China Growth Fund Class R6 $ 0.94 % |
| Expenses Paid, Amount | $ 99 |
| Expense Ratio, Percent | 0.94% |
| Factors Affecting Performance [Text Block] | Management’s Discussion of Fund Performance SUMMARY OF RESULTS The Fund returned 10.46% (net of fees) for the 12 months ended December 31, 2024. The Fund underperformed its benchmark, the MSCI China All Shares Index (net), which returned 16.38%. The Fund's benchmark is also a broad measure of market performance. Underperformance versus the benchmark TOP PERFORMANCE DETRACTORS Within Financials, AIA Group Within IT, Silergy TOP PERFORMANCE CONTRIBUTORS Partly offsetting the detractors from performance was the portfolio’s sector positioning, led by an underweight to Materials and stock selection within Consumer Staples, led by Eastroc Beverage Group |
| Performance Past Does Not Indicate Future [Text] | Keep in mind that the Fund’s past performance is not a good predictor of how the Fund will perform in the future. |
| Line Graph [Table Text Block] | |
| Average Annual Return [Table Text Block] | Average Annual Total Returns 1 Year Since Inception China Growth Fund Class R6 10.46 % -16.85 % MSCI China All Shares Index (net) 16.38 % -7.83 % |
| No Deduction of Taxes [Text Block] | The graph and table do not reflect the deduction of taxes that a shareholder would pay on Fund distributions or redemption of Fund shares. |
| Updated Performance Information Location [Text Block] | Visit www.williamblairfunds.com/funds/total-returns |
| Net Assets | $ 1,728,000 |
| Holdings Count | Holding | 45 |
| Advisory Fees Paid, Amount | $ 0 |
| Investment Company, Portfolio Turnover | 81% |
| Additional Fund Statistics [Text Block] | Key Fund Statistics Fund net assets (in $000s) $ Number of portfolio holdings 45 Net advisory fees paid (in $000s) $ Portfolio turnover rate 81 % |
| Holdings [Text Block] | Graphical Representation of Holdings The tables below show the investment makeup of the Fund, representing percentage of the total net assets of the Fund. Top Ten Holdings Tencent Holdings Ltd. 13.4 % Kweichow Moutai Co. Ltd. 5.0 % Meituan 4.5 % Contemporary Amperex Technology Co. Ltd. 4.5 % Trip.com Group Ltd. 4.2 % Xiaomi Corporation 4.1 % East Money Information Co.,Ltd. 3.3 % BYD Company Limited 3.1 % Eastroc Beverage Group Co. Ltd. 2.9 % Yutong Bus Co. Ltd. 2.8 % Sector Allocation Geographic Diversification |
| Largest Holdings [Text Block] | Top Ten Holdings Tencent Holdings Ltd. 13.4 % Kweichow Moutai Co. Ltd. 5.0 % Meituan 4.5 % Contemporary Amperex Technology Co. Ltd. 4.5 % Trip.com Group Ltd. 4.2 % Xiaomi Corporation 4.1 % East Money Information Co.,Ltd. 3.3 % BYD Company Limited 3.1 % Eastroc Beverage Group Co. Ltd. 2.9 % Yutong Bus Co. Ltd. 2.8 % |
| William Blair Emerging Markets Debt Fund Class I | |
| Shareholder Report [Line Items] | |
| Fund Name | William Blair Emerging MarketsDebt Fund |
| Class Name | Class I |
| Trading Symbol | WEDIX |
| Annual or Semi-Annual Statement [Text Block] | This annual shareholder report contains important information about William Blair Emerging Markets Debt Fund Class I (the "Fund”) for the period of January 1, 2024, to December 31, 2024. |
| Shareholder Report Annual or Semi-Annual | annual shareholder report |
| Additional Information [Text Block] | You can find additional information about the Fund at https://www.williamblairfunds.com/literature/forms |
| Additional Information Phone Number | 1-800-742-7272 |
| Additional Information Website | https://www.williamblairfunds.com/literature/forms |
| Expenses [Text Block] | What were the Fund costs for the last year? (Based on a hypothetical $10,000 investment) Fund Cost of $10,000 Investment Cost of $10,000 Investment as a percentage Emerging Markets Debt Fund Class I $ 0.70 % |
| Expenses Paid, Amount | $ 73 |
| Expense Ratio, Percent | 0.70% |
| Factors Affecting Performance [Text Block] | Management’s Discussion of Fund Performance SUMMARY OF RESULTS The Fund returned 9.06% (net of fees) for the 12 months ended December 31, 2024. The Fund outperformed its benchmark, the J.P. Morgan Emerging Markets Bond Index (EMBI) Global Diversified Index (the "Index"), which returned 6.54% and outperformed the Bloomberg Global Aggregate Index, a broad measure of market performance, which returned -1.69%. Positive contributions came from all three “beta buckets,” (or risk categories) particularly the high-beta (higher-risk ) bucket. For this group, positive performance was the result of country allocation. Performance in the medium-beta (medium-risk) bucket was driven predominantly by positive security selection and in the low-beta (lower-risk) bucket was the result of both country allocation and security selection. Higher-Risk Countries Within the higher-risk country segment, overweight positioning contributed to outperformance, especially in Ukraine and Argentina. Conversely, positions in Venezuela and Ethiopia detracted from relative performance. In Ukraine In Argentina In Venezuela In Ethiopia Lower-Risk Countries Within the lower-risk country segment, positions in China and Saudi Arabia contributed to performance. Conversely, positions in Paraguay and Trinidad and Tobago detracted from performance. In China In Saudi Arabia In Paraguay In Trinidad and Tobago Medium-Risk Countries Within the medium-risk country segment, positions in Turkey and Mexico contributed to performance. Conversely, positions in Uzbekistan and Montenegro detracted from performance. In Turkey In Mexico In Uzbekistan We lost a little performance through our marginal underweight in Montenegro |
| Performance Past Does Not Indicate Future [Text] | Keep in mind that the Fund’s past performance is not a good predictor of how the Fund will perform in the future. |
| Line Graph [Table Text Block] | |
| Average Annual Return [Table Text Block] | Average Annual Total Returns 1 Year Since Inception Emerging Markets Debt Fund Class I 9.06 % 0.36 % Bloomberg Global Aggregate Index -1.69 % -4.42 % J.P. Morgan Emerging Markets Bond Index (EMBI) Global Diversified Index 6.54 % -0.82 % |
| No Deduction of Taxes [Text Block] | The graph and table do not reflect the deduction of taxes that a shareholder would pay on Fund distributions or redemption of Fund shares. |
| Updated Performance Information Location [Text Block] | Visit www.williamblairfunds.com/funds/total-returns |
| Net Assets | $ 66,091,000 |
| Holdings Count | Holding | 245 |
| Advisory Fees Paid, Amount | $ 160,000 |
| Investment Company, Portfolio Turnover | 104% |
| Additional Fund Statistics [Text Block] | Key Fund Statistics Fund net assets (in $000s) $ Number of portfolio holdings 245 Net advisory fees paid (in $000s) $ Portfolio turnover rate 104 % |
| Holdings [Text Block] | Graphical Representation of Holdings The tables below show the investment makeup of the Fund, representing percentage of the total net assets of the Fund. Credit quality designations range from AAA (highest) to D (lowest). Credit quality ratings on underlying securities are received from S&P and Moody’s which are converted to the equivalent S & Top Ten Holdings Argentine Government International Bond 3.1 % Egypt Government International Bond 2.9 % Petroleos Mexicanos 2.8 % Qatar Government International Bond 2.4 % Paraguay Government International Bond 2.1 % Romania Government International Bond 2.1 % Dominican Republic International Bond 2.1 % Ukraine Government International Bond 2.0 % South Africa Government International Bond 1.9 % Oman Government International Bond 1.8 % Geographic Diversification Credit Quality |
| Largest Holdings [Text Block] | Top Ten Holdings Argentine Government International Bond 3.1 % Egypt Government International Bond 2.9 % Petroleos Mexicanos 2.8 % Qatar Government International Bond 2.4 % Paraguay Government International Bond 2.1 % Romania Government International Bond 2.1 % Dominican Republic International Bond 2.1 % Ukraine Government International Bond 2.0 % South Africa Government International Bond 1.9 % Oman Government International Bond 1.8 % |
| William Blair Emerging Markets Debt Fund Class R6 | |
| Shareholder Report [Line Items] | |
| Fund Name | William Blair Emerging MarketsDebt Fund |
| Class Name | Class R6 |
| Trading Symbol | WEDRX |
| Annual or Semi-Annual Statement [Text Block] | This annual shareholder report contains important information about William Blair Emerging Markets Debt Fund Class R6 (the "Fund”) for the period of January 1, 2024, to December 31, 2024. |
| Shareholder Report Annual or Semi-Annual | annual shareholder report |
| Additional Information [Text Block] | You can find additional information about the Fund at https://www.williamblairfunds.com/literature/forms |
| Additional Information Phone Number | 1-800-742-7272 |
| Additional Information Website | https://www.williamblairfunds.com/literature/forms |
| Expenses [Text Block] | What were the Fund costs for the last year? (Based on a hypothetical $10,000 investment) Fund Cost of $10,000 Investment Cost of $10,000 Investment as a percentage Emerging Markets Debt Fund Class R6 $ 0.65 % |
| Expenses Paid, Amount | $ 68 |
| Expense Ratio, Percent | 0.65% |
| Factors Affecting Performance [Text Block] | Management’s Discussion of Fund Performance SUMMARY OF RESULTS The Fund returned 9.14% (net of fees) for the 12 months ended December 31, 2024. The Fund outperformed its benchmark, the J.P. Morgan Emerging Markets Bond Index (EMBI) Global Diversified Index (the "Index"), which returned 6.54% and outperformed the Bloomberg Global Aggregate Index, a broad measure of market performance, which returned -1.69%. Positive contributions Higher-Risk Countries Within the higher-risk country segment, overweight positioning contributed to outperformance, especially in Ukraine and Argentina. Conversely, positions in Venezuela and Ethiopia detracted from relative performance. In Ukraine In Argentina In Venezuela In Ethiopia Lower-Risk Countries Within the lower-risk country segment, positions in China and Saudi Arabia contributed to performance. Conversely, positions in Paraguay and Trinidad and Tobago detracted from performance. In China In Saudi Arabia In Paraguay In Trinidad and Tobago Medium-Risk Countries Within the medium-risk country segment, positions in Turkey and Mexico contributed to performance. Conversely, positions in Uzbekistan and Montenegro detracted from performance. In Turkey In Mexico In Uzbekistan We lost a little performance through our marginal underweight in Montenegro |
| Performance Past Does Not Indicate Future [Text] | Keep in mind that the Fund’s past performance is not a good predictor of how the Fund will perform in the future. |
| Line Graph [Table Text Block] | |
| Average Annual Return [Table Text Block] | Average Annual Total Returns 1 Year Since Inception Emerging Markets Debt Fund Class R6 9.14 % 0.41 % Bloomberg Global Aggregate Index -1.69 % -4.42 % J.P. Morgan Emerging Markets Bond Index (EMBI) Global Diversified Index 6.54 % -0.82 % |
| No Deduction of Taxes [Text Block] | The graph and table do not reflect the deduction of taxes that a shareholder would pay on Fund distributions or redemption of Fund shares. |
| Updated Performance Information Location [Text Block] | Visit www.williamblairfunds.com/funds/total-returns |
| Net Assets | $ 66,091,000 |
| Holdings Count | Holding | 245 |
| Advisory Fees Paid, Amount | $ 160,000 |
| Investment Company, Portfolio Turnover | 104% |
| Additional Fund Statistics [Text Block] | Key Fund Statistics Fund net assets (in $000s) $ Number of portfolio holdings 245 Net advisory fees paid (in $000s) $ Portfolio turnover rate 104 % |
| Holdings [Text Block] | Graphical Representation of Holdings The tables below show the investment makeup of the Fund, representing percentage of the total net assets of the Fund. Credit quality designations range from AAA (highest) to D (lowest). Credit quality ratings on underlying securities are received from S&P and Moody’s which are converted to the equivalent S&P major rating category for presentation purposes only. Top Ten Holdings Argentine Government International Bond 3.1 % Egypt Government International Bond 2.9 % Petroleos Mexicanos 2.8 % Qatar Government International Bond 2.4 % Paraguay Government International Bond 2.1 % Romania Government International Bond 2.1 % Dominican Republic International Bond 2.1 % Ukraine Government International Bond 2.0 % South Africa Government International Bond 1.9 % Oman Government International Bond 1.8 % Geographic Diversification Credit Quality |
| Largest Holdings [Text Block] | Top Ten Holdings Argentine Government International Bond 3.1 % Egypt Government International Bond 2.9 % Petroleos Mexicanos 2.8 % Qatar Government International Bond 2.4 % Paraguay Government International Bond 2.1 % Romania Government International Bond 2.1 % Dominican Republic International Bond 2.1 % Ukraine Government International Bond 2.0 % South Africa Government International Bond 1.9 % Oman Government International Bond 1.8 % |
| Shareholder Report, Line Graph (Details) - USD ($) | 12 Months Ended | |||||||||||||||||||||||||
| Dec. 31, 2024 | Sep. 30, 2024 | Jun. 30, 2024 | Mar. 31, 2024 | Dec. 31, 2023 | Sep. 30, 2023 | Aug. 17, 2023 | Jun. 30, 2023 | Dec. 31, 2022 | Jul. 29, 2022 | Jun. 30, 2022 | Mar. 16, 2022 | Dec. 31, 2021 | Aug. 27, 2021 | Jul. 17, 2021 | Jun. 30, 2021 | May 25, 2021 | Dec. 31, 2020 | Dec. 31, 2019 | Oct. 01, 2019 | May 02, 2019 | Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015 | Dec. 31, 2014 | |
| William Blair Growth Fund - Class I | ||||||||||||||||||||||||||
| Account Value [Line Items] | ||||||||||||||||||||||||||
| Line Graph and Table Measure Name | Growth Fund Class I | |||||||||||||||||||||||||
| Account Value | $ 1,814,000 | $ 1,484,350 | $ 1,074,450 | $ 1,524,450 | $ 1,244,100 | $ 912,400 | $ 689,550 | $ 654,100 | $ 524,750 | $ 528,450 | $ 500,000 | |||||||||||||||
| William Blair Growth Fund - Class N | ||||||||||||||||||||||||||
| Account Value [Line Items] | ||||||||||||||||||||||||||
| Line Graph and Table Measure Name | Growth Fund Class N | |||||||||||||||||||||||||
| Account Value | $ 35,248 | 28,937 | 21,005 | 29,857 | 24,455 | 17,985 | 13,628 | 12,966 | 10,428 | 10,531 | 10,000 | |||||||||||||||
| William Blair Growth Fund - Class R6 | ||||||||||||||||||||||||||
| Account Value [Line Items] | ||||||||||||||||||||||||||
| Line Graph and Table Measure Name | Growth Fund Class R6 | |||||||||||||||||||||||||
| Account Value | $ 2,208,500 | 1,806,400 | 1,307,200 | 1,852,500 | 1,511,700 | 1,107,500 | $ 1,000,000 | |||||||||||||||||||
| William Blair Large Cap Growth Fund - Class 1 | ||||||||||||||||||||||||||
| Account Value [Line Items] | ||||||||||||||||||||||||||
| Line Graph and Table Measure Name | Large Cap Growth Fund Class I | |||||||||||||||||||||||||
| Account Value | $ 2,166,300 | 1,718,150 | 1,223,100 | 1,810,850 | 1,410,400 | 1,032,550 | 757,300 | 719,750 | 548,200 | 536,300 | 500,000 | |||||||||||||||
| William Blair Large Cap Growth Fund - Class N | ||||||||||||||||||||||||||
| Account Value [Line Items] | ||||||||||||||||||||||||||
| Line Graph and Table Measure Name | Large Cap Growth Fund Class N | |||||||||||||||||||||||||
| Account Value | $ 42,265 | 33,600 | 23,981 | 35,588 | 27,797 | 20,394 | 14,995 | 14,286 | 10,915 | 10,704 | 10,000 | |||||||||||||||
| William Blair Large Cap Growth Fund - Class R6 | ||||||||||||||||||||||||||
| Account Value [Line Items] | ||||||||||||||||||||||||||
| Line Graph and Table Measure Name | Large Cap Growth Fund Class R6 | |||||||||||||||||||||||||
| Account Value | $ 2,401,200 | 1,903,600 | 1,354,300 | 2,003,700 | 1,560,200 | 1,141,300 | 1,000,000 | |||||||||||||||||||
| William Blair Mid Cap Value Fund - Class I | ||||||||||||||||||||||||||
| Account Value [Line Items] | ||||||||||||||||||||||||||
| Line Graph and Table Measure Name | Mid Cap Value Fund Class I | |||||||||||||||||||||||||
| Account Value | $ 582,000 | $ 534,400 | 525,700 | $ 491,650 | 464,350 | $ 437,000 | $ 500,000 | |||||||||||||||||||
| William Blair Mid Cap Value Fund - Class R6 | ||||||||||||||||||||||||||
| Account Value [Line Items] | ||||||||||||||||||||||||||
| Line Graph and Table Measure Name | Mid Cap Value Fund Class R6 | |||||||||||||||||||||||||
| Account Value | $ 1,165,500 | 1,069,600 | 1,052,200 | 983,500 | 929,900 | 874,000 | 1,000,000 | |||||||||||||||||||
| William Blair Small-Mid Cap Core Fund - Class I | ||||||||||||||||||||||||||
| Account Value [Line Items] | ||||||||||||||||||||||||||
| Line Graph and Table Measure Name | Small-Mid Cap Core Fund Class I | |||||||||||||||||||||||||
| Account Value | $ 858,350 | 766,100 | 676,400 | 816,000 | 644,400 | 534,350 | $ 500,000 | |||||||||||||||||||
| William Blair Small-Mid Cap Core Fund - Class R6 | ||||||||||||||||||||||||||
| Account Value [Line Items] | ||||||||||||||||||||||||||
| Line Graph and Table Measure Name | Small-Mid Cap Core Fund Class R6 | |||||||||||||||||||||||||
| Account Value | $ 1,720,600 | 1,534,100 | 1,354,000 | 1,633,200 | 1,289,000 | 1,068,800 | 1,000,000 | |||||||||||||||||||
| William Blair Small-Mid Cap Growth Fund - Class I | ||||||||||||||||||||||||||
| Account Value [Line Items] | ||||||||||||||||||||||||||
| Line Graph and Table Measure Name | Small-Mid Cap Growth Fund Class I | |||||||||||||||||||||||||
| Account Value | $ 1,343,750 | 1,205,050 | 1,021,850 | 1,325,650 | 1,221,200 | 922,650 | 705,550 | 720,400 | 558,900 | 523,700 | 500,000 | |||||||||||||||
| William Blair Small-Mid Cap Growth Fund - Class N | ||||||||||||||||||||||||||
| Account Value [Line Items] | ||||||||||||||||||||||||||
| Line Graph and Table Measure Name | Small-Mid Cap Growth Fund Class N | |||||||||||||||||||||||||
| Account Value | $ 26,200 | 23,558 | 20,026 | 26,045 | 24,056 | 18,218 | 13,971 | 14,297 | 11,121 | 10,447 | 10,000 | |||||||||||||||
| William Blair Small-Mid Cap Growth Fund - Class R6 | ||||||||||||||||||||||||||
| Account Value [Line Items] | ||||||||||||||||||||||||||
| Line Graph and Table Measure Name | Small-Mid Cap Growth Fund Class R6 | |||||||||||||||||||||||||
| Account Value | $ 1,579,300 | 1,415,900 | 1,200,400 | 1,555,800 | 1,432,600 | 1,081,700 | 1,000,000 | |||||||||||||||||||
| William Blair Small-Mid Cap Value Fund - Class I | ||||||||||||||||||||||||||
| Account Value [Line Items] | ||||||||||||||||||||||||||
| Line Graph and Table Measure Name | Small-Mid Cap Value Fund Class I | |||||||||||||||||||||||||
| Account Value | $ 560,950 | $ 569,950 | 528,250 | $ 562,400 | 532,250 | $ 484,500 | $ 500,000 | |||||||||||||||||||
| William Blair Small-Mid Cap Value Fund - Class R6 | ||||||||||||||||||||||||||
| Account Value [Line Items] | ||||||||||||||||||||||||||
| Line Graph and Table Measure Name | Small-Mid Cap Value Fund Class R6 | |||||||||||||||||||||||||
| Account Value | $ 1,122,200 | 1,140,400 | 1,057,000 | 1,124,300 | 1,065,000 | 969,000 | 1,000,000 | |||||||||||||||||||
| William Blair Small Cap Growth Fund - Class 1 | ||||||||||||||||||||||||||
| Account Value [Line Items] | ||||||||||||||||||||||||||
| Line Graph and Table Measure Name | Small Cap Growth Fund Class I | |||||||||||||||||||||||||
| Account Value | $ 1,494,150 | 1,251,800 | 1,075,950 | 1,368,800 | 1,208,950 | 871,750 | 711,550 | 725,150 | 571,050 | 479,000 | 500,000 | |||||||||||||||
| William Blair Small Cap Growth Fund - Class N | ||||||||||||||||||||||||||
| Account Value [Line Items] | ||||||||||||||||||||||||||
| Line Graph and Table Measure Name | Small Cap Growth Fund Class N | |||||||||||||||||||||||||
| Account Value | $ 29,142 | 24,475 | 21,088 | 26,894 | 23,819 | 17,220 | 14,085 | 14,393 | 11,360 | 9,556 | 10,000 | |||||||||||||||
| William Blair Small Cap Growth Fund - Class R6 | ||||||||||||||||||||||||||
| Account Value [Line Items] | ||||||||||||||||||||||||||
| Line Graph and Table Measure Name | Small Cap Growth Fund Class R6 | |||||||||||||||||||||||||
| Account Value | $ 1,766,600 | 1,479,500 | 1,270,600 | 1,615,500 | 1,425,700 | 1,027,500 | 1,000,000 | |||||||||||||||||||
| William Blair Small Cap value fund - class I | ||||||||||||||||||||||||||
| Account Value [Line Items] | ||||||||||||||||||||||||||
| Line Graph and Table Measure Name | Small Cap Value Fund Class I | |||||||||||||||||||||||||
| Account Value | $ 1,059,200 | 1,023,950 | 923,650 | 1,039,200 | 802,550 | 780,700 | 618,850 | 719,250 | 635,750 | 484,000 | 500,000 | |||||||||||||||
| William Blair Small Cap value fund - class N | ||||||||||||||||||||||||||
| Account Value [Line Items] | ||||||||||||||||||||||||||
| Line Graph and Table Measure Name | Small Cap Value Fund Class N | |||||||||||||||||||||||||
| Account Value | $ 10,987 | 10,427 | 10,645 | 9,810 | 9,626 | 9,190 | 10,859 | $ 10,000 | ||||||||||||||||||
| William Blair Small Cap Value Fund - Class R6 [Member] | ||||||||||||||||||||||||||
| Account Value [Line Items] | ||||||||||||||||||||||||||
| Line Graph and Table Measure Name | Small Cap Value Fund Class R6 | |||||||||||||||||||||||||
| Account Value | $ 1,111,800 | 1,053,400 | 1,073,600 | 987,700 | 967,500 | 922,600 | 1,087,800 | 1,000,000 | ||||||||||||||||||
| William Blair Global Leaders Fund - Class I | ||||||||||||||||||||||||||
| Account Value [Line Items] | ||||||||||||||||||||||||||
| Line Graph and Table Measure Name | Global Leaders Fund Class I | |||||||||||||||||||||||||
| Account Value | $ 1,186,950 | 1,102,000 | 896,050 | 1,266,950 | 1,084,900 | 822,800 | 623,500 | 678,150 | 518,900 | 513,850 | 500,000 | |||||||||||||||
| William Blair Global Leaders Fund - Class N | ||||||||||||||||||||||||||
| Account Value [Line Items] | ||||||||||||||||||||||||||
| Line Graph and Table Measure Name | Global Leaders Fund Class N | |||||||||||||||||||||||||
| Account Value | $ 23,117 | 21,510 | 17,535 | 24,869 | 21,337 | 16,226 | 12,333 | 13,438 | 10,313 | 10,249 | 10,000 | |||||||||||||||
| William Blair Global Leaders Fund - Class R6 | ||||||||||||||||||||||||||
| Account Value [Line Items] | ||||||||||||||||||||||||||
| Line Graph and Table Measure Name | Global Leaders Fund Class R6 | |||||||||||||||||||||||||
| Account Value | $ 2,387,700 | 2,215,400 | 1,799,300 | 2,545,100 | 2,177,200 | 1,650,500 | 1,250,200 | 1,358,900 | 1,039,100 | 1,028,500 | 1,000,000 | |||||||||||||||
| William Blair International Leaders Fund - Class I | ||||||||||||||||||||||||||
| Account Value [Line Items] | ||||||||||||||||||||||||||
| Line Graph and Table Measure Name | International Leaders Fund Class I | |||||||||||||||||||||||||
| Account Value | $ 904,200 | 908,800 | 803,600 | 1,124,650 | 1,020,750 | 805,200 | 611,100 | 698,000 | 536,700 | 530,900 | 500,000 | |||||||||||||||
| William Blair International Leaders Fund - Class N | ||||||||||||||||||||||||||
| Account Value [Line Items] | ||||||||||||||||||||||||||
| Line Graph and Table Measure Name | International Leaders Fund Class N | |||||||||||||||||||||||||
| Account Value | $ 17,652 | 17,786 | 15,772 | 22,120 | 20,121 | 15,912 | 12,104 | 13,865 | 10,695 | 10,601 | 10,000 | |||||||||||||||
| William Blair International Leaders Fund Class R6 | ||||||||||||||||||||||||||
| Account Value [Line Items] | ||||||||||||||||||||||||||
| Line Graph and Table Measure Name | International Leaders Fund Class R6 | |||||||||||||||||||||||||
| Account Value | $ 1,821,500 | 1,830,800 | 1,617,400 | 2,262,200 | 2,052,400 | 1,617,600 | 1,227,100 | 1,400,400 | 1,076,600 | 1,063,300 | 1,000,000 | |||||||||||||||
| William Blair International Growth Fund - Class I | ||||||||||||||||||||||||||
| Account Value [Line Items] | ||||||||||||||||||||||||||
| Line Graph and Table Measure Name | International Growth Fund Class I | |||||||||||||||||||||||||
| Account Value | $ 826,100 | 806,850 | 699,300 | 975,650 | 895,000 | 678,050 | 518,950 | 630,800 | 487,150 | 499,850 | 500,000 | |||||||||||||||
| William Blair International Growth Fund - Class N | ||||||||||||||||||||||||||
| Account Value [Line Items] | ||||||||||||||||||||||||||
| Line Graph and Table Measure Name | International Growth Fund Class N | |||||||||||||||||||||||||
| Account Value | $ 16,057 | 15,726 | 13,661 | 19,109 | 17,583 | 13,357 | 10,255 | 12,506 | 9,686 | 9,973 | 10,000 | |||||||||||||||
| William Blair International Growth Fund - Class R6 | ||||||||||||||||||||||||||
| Account Value [Line Items] | ||||||||||||||||||||||||||
| Line Graph and Table Measure Name | International Growth Fund Class R6 | |||||||||||||||||||||||||
| Account Value | $ 1,365,900 | 1,333,400 | 1,154,800 | 1,610,600 | 1,476,500 | 1,117,100 | 1,000,000 | |||||||||||||||||||
| William Blair Institutional International Growth Fund - Institutional Class | ||||||||||||||||||||||||||
| Account Value [Line Items] | ||||||||||||||||||||||||||
| Line Graph and Table Measure Name | Institutional International Growth Fund | |||||||||||||||||||||||||
| Account Value | $ 8,367,000 | 8,170,500 | 7,092,500 | 9,889,500 | 9,041,000 | 6,825,000 | 5,220,000 | 6,327,500 | 4,884,500 | 5,004,500 | 5,000,000 | |||||||||||||||
| William Blair International Small Cap Growth Fund Class I | ||||||||||||||||||||||||||
| Account Value [Line Items] | ||||||||||||||||||||||||||
| Line Graph and Table Measure Name | International Small Cap Growth Fund Class I | |||||||||||||||||||||||||
| Account Value | $ 764,250 | 758,200 | 661,050 | 1,016,850 | 914,700 | 708,900 | 528,150 | 697,650 | 525,750 | 550,000 | 500,000 | |||||||||||||||
| William Blair International Small Cap Growth Fund - Class N | ||||||||||||||||||||||||||
| Account Value [Line Items] | ||||||||||||||||||||||||||
| Line Graph and Table Measure Name | International Small Cap Growth Fund Class N | |||||||||||||||||||||||||
| Account Value | $ 14,876 | 14,794 | 12,929 | 19,936 | 17,981 | 13,974 | 10,443 | 13,827 | 10,462 | 10,967 | 10,000 | |||||||||||||||
| William Blair International Small Cap Growth Fund - Class R6 | ||||||||||||||||||||||||||
| Account Value [Line Items] | ||||||||||||||||||||||||||
| Line Graph and Table Measure Name | International Small Cap Growth Fund Class R6 | |||||||||||||||||||||||||
| Account Value | $ 1,540,600 | 1,527,500 | 1,331,100 | 2,045,800 | 1,838,600 | 1,422,800 | 1,059,300 | 1,397,300 | 1,052,900 | 1,100,300 | 1,000,000 | |||||||||||||||
| William Blair Emerging Markets Leaders Fund - Class I | ||||||||||||||||||||||||||
| Account Value [Line Items] | ||||||||||||||||||||||||||
| Line Graph and Table Measure Name | Emerging Markets Leaders Fund Class I | |||||||||||||||||||||||||
| Account Value | $ 644,250 | 608,650 | 573,450 | 773,300 | 835,750 | 655,400 | 510,600 | 618,550 | 435,950 | 428,150 | 500,000 | |||||||||||||||
| William Blair Emerging Markets Leaders Fund - Class N | ||||||||||||||||||||||||||
| Account Value [Line Items] | ||||||||||||||||||||||||||
| Line Graph and Table Measure Name | Emerging Markets Leaders Fund Class N | |||||||||||||||||||||||||
| Account Value | $ 12,551 | 11,875 | 11,219 | 15,182 | 16,446 | 12,926 | 10,100 | 12,277 | 8,665 | 8,544 | 10,000 | |||||||||||||||
| William Blair Emerging Markets Leaders Fund - Class R6 | ||||||||||||||||||||||||||
| Account Value [Line Items] | ||||||||||||||||||||||||||
| Line Graph and Table Measure Name | Emerging Markets Leaders Fund Class R6 | |||||||||||||||||||||||||
| Account Value | $ 1,295,600 | 1,223,100 | 1,151,500 | 1,553,300 | 1,676,400 | 1,314,900 | 1,023,600 | 1,240,200 | 872,500 | 857,600 | 1,000,000 | |||||||||||||||
| William Blair Emerging Markets Growth Fund - Class I | ||||||||||||||||||||||||||
| Account Value [Line Items] | ||||||||||||||||||||||||||
| Line Graph and Table Measure Name | Emerging Markets Growth Fund Class I | |||||||||||||||||||||||||
| Account Value | $ 770,350 | 696,400 | 631,750 | 944,950 | 915,250 | 650,400 | 507,000 | 644,700 | 433,200 | 425,850 | 500,000 | |||||||||||||||
| William Blair Emerging Markets Growth Fund - Class N | ||||||||||||||||||||||||||
| Account Value [Line Items] | ||||||||||||||||||||||||||
| Line Graph and Table Measure Name | Emerging Markets Growth Fund Class N | |||||||||||||||||||||||||
| Account Value | $ 15,031 | 13,615 | 12,379 | 18,569 | 18,033 | 12,841 | 10,041 | 12,809 | 8,624 | 8,497 | 10,000 | |||||||||||||||
| William Blair Emerging Markets Growth Fund - Class R6 | ||||||||||||||||||||||||||
| Account Value [Line Items] | ||||||||||||||||||||||||||
| Line Graph and Table Measure Name | Emerging Markets Growth Fund Class R6 | |||||||||||||||||||||||||
| Account Value | $ 1,553,200 | 1,402,900 | 1,272,400 | 1,902,700 | 1,840,700 | 1,306,300 | 1,018,300 | 1,293,700 | 867,900 | 853,300 | 1,000,000 | |||||||||||||||
| William Blair Emerging Markets ex China Growth Fund - Class 1 | ||||||||||||||||||||||||||
| Account Value [Line Items] | ||||||||||||||||||||||||||
| Line Graph and Table Measure Name | Emerging Markets ex China Growth Fund Class I | |||||||||||||||||||||||||
| Account Value | $ 666,100 | 668,150 | 586,950 | 534,000 | 470,500 | $ 500,000 | ||||||||||||||||||||
| William Blair Emerging Markets ex China Growth Fund - Class R6 | ||||||||||||||||||||||||||
| Account Value [Line Items] | ||||||||||||||||||||||||||
| Line Graph and Table Measure Name | Emerging Markets ex China Growth Fund Class R6 | |||||||||||||||||||||||||
| Account Value | $ 1,334,600 | 1,338,600 | 1,175,200 | 1,069,000 | 942,000 | 1,000,000 | ||||||||||||||||||||
| William Blair Emerging Markets Small Cap Growth Fund - Class 1 | ||||||||||||||||||||||||||
| Account Value [Line Items] | ||||||||||||||||||||||||||
| Line Graph and Table Measure Name | Emerging Markets Small Cap Growth Fund Class I | |||||||||||||||||||||||||
| Account Value | $ 847,350 | 781,900 | 639,600 | 887,450 | 768,300 | 579,400 | 480,550 | 626,550 | 445,850 | 469,850 | 500,000 | |||||||||||||||
| William Blair Emerging Markets Small Cap Growth Fund - Class N | ||||||||||||||||||||||||||
| Account Value [Line Items] | ||||||||||||||||||||||||||
| Line Graph and Table Measure Name | Emerging Markets Small Cap Growth Fund Class N | |||||||||||||||||||||||||
| Account Value | $ 16,486 | 15,252 | 12,508 | 17,402 | 15,101 | 11,418 | 9,488 | 12,413 | 8,861 | 9,360 | 10,000 | |||||||||||||||
| William Blair Emerging Markets Small Cap Growth Fund - Class R6 | ||||||||||||||||||||||||||
| Account Value [Line Items] | ||||||||||||||||||||||||||
| Line Graph and Table Measure Name | Emerging Markets Small Cap Growth Fund Class R6 | |||||||||||||||||||||||||
| Account Value | $ 1,705,100 | 1,573,100 | 1,285,500 | 1,783,200 | 1,542,900 | 1,163,000 | 963,700 | 1,255,500 | 892,800 | 940,300 | 1,000,000 | |||||||||||||||
| William Blair China Growth Fund - Class 1 | ||||||||||||||||||||||||||
| Account Value [Line Items] | ||||||||||||||||||||||||||
| Line Graph and Table Measure Name | China Growth Fund Class I | |||||||||||||||||||||||||
| Account Value | $ 269,850 | 252,600 | 244,550 | 274,500 | 323,000 | 388,500 | 478,000 | $ 500,000 | ||||||||||||||||||
| William Blair China Growth Fund - Class R6 | ||||||||||||||||||||||||||
| Account Value [Line Items] | ||||||||||||||||||||||||||
| Line Graph and Table Measure Name | China Growth Fund Class R6 | |||||||||||||||||||||||||
| Account Value | $ 539,100 | 504,200 | 488,100 | 547,400 | 645,400 | 777,000 | 956,000 | 1,000,000 | ||||||||||||||||||
| William Blair Emerging Markets Debt Fund Class I | ||||||||||||||||||||||||||
| Account Value [Line Items] | ||||||||||||||||||||||||||
| Line Graph and Table Measure Name | Emerging Markets Debt Fund Class I | |||||||||||||||||||||||||
| Account Value | $ 506,800 | 483,150 | 464,400 | 430,450 | 409,400 | 391,100 | 493,000 | $ 501,850 | $ 500,000 | |||||||||||||||||
| William Blair Emerging Markets Debt Fund Class R6 | ||||||||||||||||||||||||||
| Account Value [Line Items] | ||||||||||||||||||||||||||
| Line Graph and Table Measure Name | Emerging Markets Debt Fund Class R6 | |||||||||||||||||||||||||
| Account Value | $ 1,015,300 | 967,500 | 929,700 | 861,400 | 819,200 | 782,900 | 986,600 | 1,003,700 | 1,000,000 | |||||||||||||||||
| Standard & Poor's 500 Index [Member] | ||||||||||||||||||||||||||
| Account Value [Line Items] | ||||||||||||||||||||||||||
| Line Graph and Table Measure Name | S&P 500® Index | |||||||||||||||||||||||||
| Account Value | $ 1,712,700 | 1,369,950 | 1,084,800 | 1,324,700 | 1,029,250 | 869,300 | 1,000,000 | 661,150 | 691,450 | 567,550 | 506,900 | 500,000 | ||||||||||||||
| Russell 1000® Index [Member] | ||||||||||||||||||||||||||
| Account Value [Line Items] | ||||||||||||||||||||||||||
| Line Graph and Table Measure Name | Russell 1000® Index | |||||||||||||||||||||||||
| Account Value | $ 1,678,000 | 1,347,650 | 1,065,100 | 1,317,000 | 1,041,500 | 861,000 | 1,000,000 | 655,100 | 688,050 | 565,400 | 504,600 | 500,000 | ||||||||||||||
| Russell 3000® Index [Member] | ||||||||||||||||||||||||||
| Account Value [Line Items] | ||||||||||||||||||||||||||
| Line Graph and Table Measure Name | Russell 3000® Index | |||||||||||||||||||||||||
| Account Value | $ 708,300 | 665,050 | 649,650 | 606,550 | 572,100 | 491,900 | 500,000 | 527,650 | 454,200 | 443,600 | 500,000 | 1,293,850 | 10,000 | 1,029,650 | 851,750 | 500,000 | 1,000,000 | 650,100 | 686,050 | 566,350 | 502,400 | 500,000 | ||||
| MSCI All Country World IMI (net) [Member] | ||||||||||||||||||||||||||
| Account Value [Line Items] | ||||||||||||||||||||||||||
| Line Graph and Table Measure Name | MSCI All Country World IMI (net) | |||||||||||||||||||||||||
| Account Value | $ 1,184,150 | 1,017,550 | 836,950 | 1,025,700 | 867,600 | 746,300 | 590,650 | 656,850 | 529,950 | 489,050 | 500,000 | |||||||||||||||
| MSCI All Country World Ex-U.S. IMI (net) [Member] | ||||||||||||||||||||||||||
| Account Value [Line Items] | ||||||||||||||||||||||||||
| Line Graph and Table Measure Name | MSCI All Country World ex-U.S. IMI (net) | |||||||||||||||||||||||||
| Account Value | $ 8,078,000 | 7,676,000 | 6,639,000 | 7,959,000 | 7,333,500 | 6,600,000 | 1,000,000 | 5,426,000 | 6,365,500 | 4,980,000 | 4,770,000 | 5,000,000 | ||||||||||||||
| MSCI Emerging Markets Index (net) [Member] | ||||||||||||||||||||||||||
| Account Value [Line Items] | ||||||||||||||||||||||||||
| Line Graph and Table Measure Name | MSCI Emerging Markets Index (net) | |||||||||||||||||||||||||
| Account Value | $ 714,650 | 664,750 | 605,250 | 757,450 | 777,200 | 656,950 | 554,750 | 649,350 | 473,000 | 425,400 | 500,000 | |||||||||||||||
| MSCI Emerging Markets IMI (net) [Member] | ||||||||||||||||||||||||||
| Account Value [Line Items] | ||||||||||||||||||||||||||
| Line Graph and Table Measure Name | MSCI Emerging Markets IMI (net) | |||||||||||||||||||||||||
| Account Value | $ 732,700 | 684,200 | 612,700 | 764,250 | 766,400 | 647,300 | 550,250 | 647,650 | 473,350 | 430,700 | 500,000 | |||||||||||||||
| MSCI Emerging Markets ex-China IMI (net) [Member] | ||||||||||||||||||||||||||
| Account Value [Line Items] | ||||||||||||||||||||||||||
| Line Graph and Table Measure Name | MSCI Emerging Markets ex-China IMI (net) | |||||||||||||||||||||||||
| Account Value | $ 621,550 | 649,550 | 599,350 | 544,400 | 492,700 | $ 500,000 | ||||||||||||||||||||
| MSCI China All Shares Index (net) [Member] | ||||||||||||||||||||||||||
| Account Value [Line Items] | ||||||||||||||||||||||||||
| Line Graph and Table Measure Name | MSCI China All Shares Index (net) | |||||||||||||||||||||||||
| Account Value | $ 380,650 | 334,500 | 327,050 | 349,900 | 369,700 | 427,000 | 483,950 | $ 500,000 | ||||||||||||||||||
| Bloomberg Global Aggregate Index [Member] | ||||||||||||||||||||||||||
| Account Value [Line Items] | ||||||||||||||||||||||||||
| Line Graph and Table Measure Name | Bloomberg Global Aggregate Index | |||||||||||||||||||||||||
| Account Value | $ 424,750 | 418,400 | 432,100 | 414,550 | 408,700 | 420,100 | 487,950 | 495,650 | 500,000 | |||||||||||||||||
| Russell 3000 Growth Index [Member] | ||||||||||||||||||||||||||
| Account Value [Line Items] | ||||||||||||||||||||||||||
| Line Graph and Table Measure Name | Russell 3000® Growth Index | |||||||||||||||||||||||||
| Account Value | $ 2,247,950 | 1,697,100 | 1,201,850 | 1,691,950 | 1,344,400 | 972,350 | 1,000,000 | 715,800 | 731,300 | 564,300 | 525,450 | 500,000 | ||||||||||||||
| Russell 1000® Growth Index [Member] | ||||||||||||||||||||||||||
| Account Value [Line Items] | ||||||||||||||||||||||||||
| Line Graph and Table Measure Name | Russell 1000® Growth Index | |||||||||||||||||||||||||
| Account Value | $ 2,357,600 | 1,767,900 | 1,239,100 | 1,748,550 | 1,370,400 | 989,500 | 1,000,000 | 725,500 | 736,650 | 565,700 | 528,350 | 500,000 | ||||||||||||||
| Russell Midcap Value Index [Member] | ||||||||||||||||||||||||||
| Account Value [Line Items] | ||||||||||||||||||||||||||
| Line Graph and Table Measure Name | Russell Midcap® Value Index | |||||||||||||||||||||||||
| Account Value | $ 598,500 | 553,350 | 529,300 | 494,150 | 469,600 | 447,200 | $ 500,000 | |||||||||||||||||||
| Russell 2500 Index [Member] | ||||||||||||||||||||||||||
| Account Value [Line Items] | ||||||||||||||||||||||||||
| Line Graph and Table Measure Name | Russell 2500™ Index | |||||||||||||||||||||||||
| Account Value | $ 826,200 | 737,700 | 628,250 | 769,600 | 651,200 | 542,700 | $ 500,000 | |||||||||||||||||||
| Russell 2500 Growth Index [Member] | ||||||||||||||||||||||||||
| Account Value [Line Items] | ||||||||||||||||||||||||||
| Line Graph and Table Measure Name | Russell 2500™ Growth Index | |||||||||||||||||||||||||
| Account Value | $ 1,233,850 | 1,083,300 | 910,850 | 1,234,400 | 1,175,150 | 836,600 | $ 1,000,000 | 630,650 | 681,550 | 547,600 | 499,050 | 500,000 | ||||||||||||||
| Russell 2500 Value Index [Member] | ||||||||||||||||||||||||||
| Account Value [Line Items] | ||||||||||||||||||||||||||
| Line Graph and Table Measure Name | Russell 2500® Value Index | |||||||||||||||||||||||||
| Account Value | $ 613,750 | $ 615,100 | 561,100 | $ 586,350 | 552,800 | $ 485,950 | $ 500,000 | |||||||||||||||||||
| Russell 2000® Growth Index [Member] | ||||||||||||||||||||||||||
| Account Value [Line Items] | ||||||||||||||||||||||||||
| Line Graph and Table Measure Name | Russell 2000® Growth Index | |||||||||||||||||||||||||
| Account Value | $ 1,088,600 | 945,300 | 796,650 | 1,081,800 | 1,052,000 | 781,400 | 608,150 | 670,550 | 548,900 | 493,100 | 500,000 | |||||||||||||||
| Russell 2000® Value Index [Member] | ||||||||||||||||||||||||||
| Account Value [Line Items] | ||||||||||||||||||||||||||
| Line Graph and Table Measure Name | Russell 2000® Value Index | |||||||||||||||||||||||||
| Account Value | $ 996,700 | 10,439 | 922,400 | 9,414 | 804,550 | 8,880 | 940,800 | $ 10,000 | 733,450 | 701,000 | 572,750 | 657,300 | 609,500 | 462,650 | 500,000 | |||||||||||
| MSCI All Country World ex-U.S. Small Cap Index (net) [Member] | ||||||||||||||||||||||||||
| Account Value [Line Items] | ||||||||||||||||||||||||||
| Line Graph and Table Measure Name | MSCI All Country World ex-U.S. Small Cap Index (net) | |||||||||||||||||||||||||
| Account Value | $ 867,300 | 839,100 | 725,500 | 906,550 | 802,800 | 702,750 | 574,050 | 701,750 | 533,050 | 513,000 | 500,000 | |||||||||||||||
| MSCI Emerging Markets Small Cap Index (net) [Member] | ||||||||||||||||||||||||||
| Account Value [Line Items] | ||||||||||||||||||||||||||
| Line Graph and Table Measure Name | MSCI Emerging Markets Small Cap Index (net) | |||||||||||||||||||||||||
| Account Value | $ 872,750 | 832,850 | 672,100 | 819,850 | $ 690,400 | $ 578,750 | $ 519,050 | $ 637,600 | $ 476,400 | $ 465,750 | $ 500,000 | |||||||||||||||
| JPMorgan Emerging Markets Bond Index (EMBI) Global Diversified (net) [Member] | ||||||||||||||||||||||||||
| Account Value [Line Items] | ||||||||||||||||||||||||||
| Account Value | $ 485,450 | $ 466,350 | $ 455,700 | $ 426,950 | $ 410,200 | $ 397,600 | $ 498,900 | $ 504,650 | $ 500,000 |
| Shareholder Report, Average Annual Return (Details) | 12 Months Ended | 16 Months Ended | 29 Months Ended | 34 Months Ended | 40 Months Ended | 41 Months Ended | 43 Months Ended | 60 Months Ended | 63 Months Ended | 68 Months Ended | 120 Months Ended |
| Dec. 31, 2024 | Dec. 31, 2024 | Dec. 31, 2024 | Dec. 31, 2024 | Dec. 31, 2024 | Dec. 31, 2024 | Dec. 31, 2024 | Dec. 31, 2024 | Dec. 31, 2024 | Dec. 31, 2024 | Dec. 31, 2024 | |
| William Blair Growth Fund - Class I | |||||||||||
| Average Annual Return [Line Items] | |||||||||||
| Line Graph and Table Measure Name | Growth Fund Class I | ||||||||||
| Average Annual Return, Percent | 22.21% | 14.73% | 13.75% | ||||||||
| William Blair Growth Fund - Class N | |||||||||||
| Average Annual Return [Line Items] | |||||||||||
| Line Graph and Table Measure Name | Growth Fund Class N | ||||||||||
| Average Annual Return, Percent | 21.81% | 14.40% | 13.43% | ||||||||
| William Blair Growth Fund - Class R6 | |||||||||||
| Average Annual Return [Line Items] | |||||||||||
| Line Graph and Table Measure Name | Growth Fund Class R6 | ||||||||||
| Average Annual Return, Percent | 22.26% | 14.80% | 15% | ||||||||
| William Blair Large Cap Growth Fund - Class 1 | |||||||||||
| Average Annual Return [Line Items] | |||||||||||
| Line Graph and Table Measure Name | Large Cap Growth Fund Class I | ||||||||||
| Average Annual Return, Percent | 26.08% | 15.97% | 15.79% | ||||||||
| William Blair Large Cap Growth Fund - Class N | |||||||||||
| Average Annual Return [Line Items] | |||||||||||
| Line Graph and Table Measure Name | Large Cap Growth Fund Class N | ||||||||||
| Average Annual Return, Percent | 25.79% | 15.69% | 15.50% | ||||||||
| William Blair Large Cap Growth Fund - Class R6 | |||||||||||
| Average Annual Return [Line Items] | |||||||||||
| Line Graph and Table Measure Name | Large Cap Growth Fund Class R6 | ||||||||||
| Average Annual Return, Percent | 26.14% | 16.04% | 16.71% | ||||||||
| William Blair Mid Cap Value Fund - Class I | |||||||||||
| Average Annual Return [Line Items] | |||||||||||
| Line Graph and Table Measure Name | Mid Cap Value Fund Class I | ||||||||||
| Average Annual Return, Percent | 10.71% | 5.57% | |||||||||
| William Blair Mid Cap Value Fund - Class R6 | |||||||||||
| Average Annual Return [Line Items] | |||||||||||
| Line Graph and Table Measure Name | Mid Cap Value Fund Class R6 | ||||||||||
| Average Annual Return, Percent | 10.77% | 5.62% | |||||||||
| William Blair Small-Mid Cap Core Fund - Class I | |||||||||||
| Average Annual Return [Line Items] | |||||||||||
| Line Graph and Table Measure Name | Small-Mid Cap Core Fund Class I | ||||||||||
| Average Annual Return, Percent | 12.04% | 9.94% | 10.84% | ||||||||
| William Blair Small-Mid Cap Core Fund - Class R6 | |||||||||||
| Average Annual Return [Line Items] | |||||||||||
| Line Graph and Table Measure Name | Small-Mid Cap Core Fund Class R6 | ||||||||||
| Average Annual Return, Percent | 12.16% | 9.99% | 10.88% | ||||||||
| William Blair Small-Mid Cap Growth Fund - Class I | |||||||||||
| Average Annual Return [Line Items] | |||||||||||
| Line Graph and Table Measure Name | Small-Mid Cap Growth Fund Class I | ||||||||||
| Average Annual Return, Percent | 11.51% | 7.81% | 10.39% | ||||||||
| William Blair Small-Mid Cap Growth Fund - Class N | |||||||||||
| Average Annual Return [Line Items] | |||||||||||
| Line Graph and Table Measure Name | Small-Mid Cap Growth Fund Class N | ||||||||||
| Average Annual Return, Percent | 11.21% | 7.54% | 10.11% | ||||||||
| William Blair Small-Mid Cap Growth Fund - Class R6 | |||||||||||
| Average Annual Return [Line Items] | |||||||||||
| Line Graph and Table Measure Name | Small-Mid Cap Growth Fund Class R6 | ||||||||||
| Average Annual Return, Percent | 11.54% | 7.86% | 8.40% | ||||||||
| William Blair Small-Mid Cap Value Fund - Class I | |||||||||||
| Average Annual Return [Line Items] | |||||||||||
| Line Graph and Table Measure Name | Small-Mid Cap Value Fund Class I | ||||||||||
| Average Annual Return, Percent | 5.39% | 8.72% | |||||||||
| William Blair Small-Mid Cap Value Fund - Class R6 | |||||||||||
| Average Annual Return [Line Items] | |||||||||||
| Line Graph and Table Measure Name | Small-Mid Cap Value Fund Class R6 | ||||||||||
| Average Annual Return, Percent | 5.37% | 8.74% | |||||||||
| William Blair Small Cap Growth Fund - Class 1 | |||||||||||
| Average Annual Return [Line Items] | |||||||||||
| Line Graph and Table Measure Name | Small Cap Growth Fund Class I | ||||||||||
| Average Annual Return, Percent | 19.36% | 11.38% | 11.57% | ||||||||
| William Blair Small Cap Growth Fund - Class N | |||||||||||
| Average Annual Return [Line Items] | |||||||||||
| Line Graph and Table Measure Name | Small Cap Growth Fund Class N | ||||||||||
| Average Annual Return, Percent | 19.07% | 11.10% | 11.29% | ||||||||
| William Blair Small Cap Growth Fund - Class R6 | |||||||||||
| Average Annual Return [Line Items] | |||||||||||
| Line Graph and Table Measure Name | Small Cap Growth Fund Class R6 | ||||||||||
| Average Annual Return, Percent | 19.41% | 11.45% | 10.56% | ||||||||
| William Blair Small Cap value fund - class I | |||||||||||
| Average Annual Return [Line Items] | |||||||||||
| Line Graph and Table Measure Name | Small Cap Value Fund Class I | ||||||||||
| Average Annual Return, Percent | 3.44% | 6.29% | 7.80% | ||||||||
| William Blair Small Cap value fund - class N | |||||||||||
| Average Annual Return [Line Items] | |||||||||||
| Line Graph and Table Measure Name | Small Cap Value Fund Class N | ||||||||||
| Average Annual Return, Percent | 3.21% | 2.76% | |||||||||
| William Blair Small Cap Value Fund - Class R6 [Member] | |||||||||||
| Average Annual Return [Line Items] | |||||||||||
| Line Graph and Table Measure Name | Small Cap Value Fund Class R6 | ||||||||||
| Average Annual Return, Percent | 3.56% | 3.12% | |||||||||
| William Blair Global Leaders Fund - Class I | |||||||||||
| Average Annual Return [Line Items] | |||||||||||
| Line Graph and Table Measure Name | Global Leaders Fund Class I | ||||||||||
| Average Annual Return, Percent | 7.71% | 7.60% | 9.03% | ||||||||
| William Blair Global Leaders Fund - Class N | |||||||||||
| Average Annual Return [Line Items] | |||||||||||
| Line Graph and Table Measure Name | Global Leaders Fund Class N | ||||||||||
| Average Annual Return, Percent | 7.47% | 7.34% | 8.74% | ||||||||
| William Blair Global Leaders Fund - Class R6 | |||||||||||
| Average Annual Return [Line Items] | |||||||||||
| Line Graph and Table Measure Name | Global Leaders Fund Class R6 | ||||||||||
| Average Annual Return, Percent | 7.78% | 7.66% | 9.09% | ||||||||
| William Blair International Leaders Fund - Class I | |||||||||||
| Average Annual Return [Line Items] | |||||||||||
| Line Graph and Table Measure Name | International Leaders Fund Class I | ||||||||||
| Average Annual Return, Percent | (0.50%) | 2.35% | 6.10% | ||||||||
| William Blair International Leaders Fund - Class N | |||||||||||
| Average Annual Return [Line Items] | |||||||||||
| Line Graph and Table Measure Name | International Leaders Fund Class N | ||||||||||
| Average Annual Return, Percent | (0.76%) | 2.10% | 5.85% | ||||||||
| William Blair International Leaders Fund Class R6 | |||||||||||
| Average Annual Return [Line Items] | |||||||||||
| Line Graph and Table Measure Name | International Leaders Fund Class R6 | ||||||||||
| Average Annual Return, Percent | (0.51%) | 2.40% | 6.18% | ||||||||
| William Blair International Growth Fund - Class I | |||||||||||
| Average Annual Return [Line Items] | |||||||||||
| Line Graph and Table Measure Name | International Growth Fund Class I | ||||||||||
| Average Annual Return, Percent | 2.38% | 4.03% | 5.15% | ||||||||
| William Blair International Growth Fund - Class N | |||||||||||
| Average Annual Return [Line Items] | |||||||||||
| Line Graph and Table Measure Name | International Growth Fund Class N | ||||||||||
| Average Annual Return, Percent | 2.11% | 3.75% | 4.85% | ||||||||
| William Blair International Growth Fund - Class R6 | |||||||||||
| Average Annual Return [Line Items] | |||||||||||
| Line Graph and Table Measure Name | International Growth Fund Class R6 | ||||||||||
| Average Annual Return, Percent | 2.43% | 4.10% | 5.65% | ||||||||
| William Blair Institutional International Growth Fund - Institutional Class | |||||||||||
| Average Annual Return [Line Items] | |||||||||||
| Line Graph and Table Measure Name | Institutional International Growth Fund | ||||||||||
| Average Annual Return, Percent | 2.41% | 4.16% | 5.28% | ||||||||
| William Blair International Small Cap Growth Fund Class I | |||||||||||
| Average Annual Return [Line Items] | |||||||||||
| Line Graph and Table Measure Name | International Small Cap Growth Fund Class I | ||||||||||
| Average Annual Return, Percent | 0.80% | 1.52% | 4.33% | ||||||||
| William Blair International Small Cap Growth Fund - Class N | |||||||||||
| Average Annual Return [Line Items] | |||||||||||
| Line Graph and Table Measure Name | International Small Cap Growth Fund Class N | ||||||||||
| Average Annual Return, Percent | 0.55% | 1.26% | 4.05% | ||||||||
| William Blair International Small Cap Growth Fund - Class R6 | |||||||||||
| Average Annual Return [Line Items] | |||||||||||
| Line Graph and Table Measure Name | International Small Cap Growth Fund Class R6 | ||||||||||
| Average Annual Return, Percent | 0.86% | 1.60% | 4.42% | ||||||||
| William Blair Emerging Markets Leaders Fund - Class I | |||||||||||
| Average Annual Return [Line Items] | |||||||||||
| Line Graph and Table Measure Name | Emerging Markets Leaders Fund Class I | ||||||||||
| Average Annual Return, Percent | 5.86% | (0.34%) | 2.57% | ||||||||
| William Blair Emerging Markets Leaders Fund - Class N | |||||||||||
| Average Annual Return [Line Items] | |||||||||||
| Line Graph and Table Measure Name | Emerging Markets Leaders Fund Class N | ||||||||||
| Average Annual Return, Percent | 5.69% | (0.59%) | 2.30% | ||||||||
| William Blair Emerging Markets Leaders Fund - Class R6 | |||||||||||
| Average Annual Return [Line Items] | |||||||||||
| Line Graph and Table Measure Name | Emerging Markets Leaders Fund Class R6 | ||||||||||
| Average Annual Return, Percent | 5.92% | (0.30%) | 2.62% | ||||||||
| William Blair Emerging Markets Growth Fund - Class I | |||||||||||
| Average Annual Return [Line Items] | |||||||||||
| Line Graph and Table Measure Name | Emerging Markets Growth Fund Class I | ||||||||||
| Average Annual Return, Percent | 10.62% | 3.44% | 4.42% | ||||||||
| William Blair Emerging Markets Growth Fund - Class N | |||||||||||
| Average Annual Return [Line Items] | |||||||||||
| Line Graph and Table Measure Name | Emerging Markets Growth Fund Class N | ||||||||||
| Average Annual Return, Percent | 10.40% | 3.20% | 4.16% | ||||||||
| William Blair Emerging Markets Growth Fund - Class R6 | |||||||||||
| Average Annual Return [Line Items] | |||||||||||
| Line Graph and Table Measure Name | Emerging Markets Growth Fund Class R6 | ||||||||||
| Average Annual Return, Percent | 10.71% | 3.52% | 4.50% | ||||||||
| William Blair Emerging Markets ex China Growth Fund - Class 1 | |||||||||||
| Average Annual Return [Line Items] | |||||||||||
| Line Graph and Table Measure Name | Emerging Markets ex China Growth Fund Class I | ||||||||||
| Average Annual Return, Percent | 13.49% | 12.53% | |||||||||
| William Blair Emerging Markets ex China Growth Fund - Class R6 | |||||||||||
| Average Annual Return [Line Items] | |||||||||||
| Line Graph and Table Measure Name | Emerging Markets ex China Growth Fund Class R6 | ||||||||||
| Average Annual Return, Percent | 13.56% | 12.61% | |||||||||
| William Blair Emerging Markets Small Cap Growth Fund - Class 1 | |||||||||||
| Average Annual Return [Line Items] | |||||||||||
| Line Graph and Table Measure Name | Emerging Markets Small Cap Growth Fund Class I | ||||||||||
| Average Annual Return, Percent | 8.37% | 7.90% | 5.42% | ||||||||
| William Blair Emerging Markets Small Cap Growth Fund - Class N | |||||||||||
| Average Annual Return [Line Items] | |||||||||||
| Line Graph and Table Measure Name | Emerging Markets Small Cap Growth Fund Class N | ||||||||||
| Average Annual Return, Percent | 8.09% | 7.62% | 5.13% | ||||||||
| William Blair Emerging Markets Small Cap Growth Fund - Class R6 | |||||||||||
| Average Annual Return [Line Items] | |||||||||||
| Line Graph and Table Measure Name | Emerging Markets Small Cap Growth Fund Class R6 | ||||||||||
| Average Annual Return, Percent | 8.39% | 7.95% | 5.48% | ||||||||
| William Blair China Growth Fund - Class 1 | |||||||||||
| Average Annual Return [Line Items] | |||||||||||
| Line Graph and Table Measure Name | China Growth Fund Class I | ||||||||||
| Average Annual Return, Percent | 10.34% | (16.82%) | |||||||||
| William Blair China Growth Fund - Class R6 | |||||||||||
| Average Annual Return [Line Items] | |||||||||||
| Line Graph and Table Measure Name | China Growth Fund Class R6 | ||||||||||
| Average Annual Return, Percent | 10.46% | (16.85%) | |||||||||
| William Blair Emerging Markets Debt Fund Class I | |||||||||||
| Average Annual Return [Line Items] | |||||||||||
| Line Graph and Table Measure Name | Emerging Markets Debt Fund Class I | ||||||||||
| Average Annual Return, Percent | 9.06% | 0.36% | |||||||||
| William Blair Emerging Markets Debt Fund Class R6 | |||||||||||
| Average Annual Return [Line Items] | |||||||||||
| Line Graph and Table Measure Name | Emerging Markets Debt Fund Class R6 | ||||||||||
| Average Annual Return, Percent | 9.14% | 0.41% | |||||||||
| Standard & Poor's 500 Index [Member] | |||||||||||
| Average Annual Return [Line Items] | |||||||||||
| Line Graph and Table Measure Name | S&P 500® Index | ||||||||||
| Average Annual Return, Percent | 25.02% | 14.53% | 15% | 13.10% | |||||||
| Russell 1000® Index [Member] | |||||||||||
| Average Annual Return [Line Items] | |||||||||||
| Line Graph and Table Measure Name | Russell 1000® Index | ||||||||||
| Average Annual Return, Percent | 24.51% | 14.28% | 14.69% | 12.87% | |||||||
| Russell 3000® Index [Member] | |||||||||||
| Average Annual Return [Line Items] | |||||||||||
| Line Graph and Table Measure Name | Russell 3000® Index | ||||||||||
| Average Annual Return, Percent | 23.81% | 25.39% | 13.25% | 9.71% | 13.86% | 15.03% | 14.26% | 12.55% | |||
| MSCI All Country World IMI (net) [Member] | |||||||||||
| Average Annual Return [Line Items] | |||||||||||
| Line Graph and Table Measure Name | MSCI All Country World IMI (net) | ||||||||||
| Average Annual Return, Percent | 16.37% | 9.67% | 9% | ||||||||
| MSCI All Country World Ex-U.S. IMI (net) [Member] | |||||||||||
| Average Annual Return [Line Items] | |||||||||||
| Line Graph and Table Measure Name | MSCI All Country World ex-U.S. IMI (net) | ||||||||||
| Average Annual Return, Percent | 5.23% | 4.12% | 4.93% | 4.91% | |||||||
| MSCI Emerging Markets Index (net) [Member] | |||||||||||
| Average Annual Return [Line Items] | |||||||||||
| Line Graph and Table Measure Name | MSCI Emerging Markets Index (net) | ||||||||||
| Average Annual Return, Percent | 7.50% | 1.70% | 3.64% | ||||||||
| MSCI Emerging Markets ex-China IMI (net) [Member] | |||||||||||
| Average Annual Return [Line Items] | |||||||||||
| Line Graph and Table Measure Name | MSCI Emerging Markets ex-China IMI (net) | ||||||||||
| Average Annual Return, Percent | 3.70% | 9.39% | |||||||||
| MSCI Emerging Markets IMI (net) [Member] | |||||||||||
| Average Annual Return [Line Items] | |||||||||||
| Line Graph and Table Measure Name | MSCI Emerging Markets IMI (net) | ||||||||||
| Average Annual Return, Percent | 7.09% | 2.51% | 3.90% | ||||||||
| MSCI China All Shares Index (net) [Member] | |||||||||||
| Average Annual Return [Line Items] | |||||||||||
| Line Graph and Table Measure Name | MSCI China All Shares Index (net) | ||||||||||
| Average Annual Return, Percent | 16.38% | (7.83%) | |||||||||
| Bloomberg Global Aggregate Index [Member] | |||||||||||
| Average Annual Return [Line Items] | |||||||||||
| Line Graph and Table Measure Name | Bloomberg Global Aggregate Index | ||||||||||
| Average Annual Return, Percent | (1.69%) | (4.42%) | |||||||||
| Russell 3000 Growth Index [Member] | |||||||||||
| Average Annual Return [Line Items] | |||||||||||
| Line Graph and Table Measure Name | Russell 3000® Growth Index | ||||||||||
| Average Annual Return, Percent | 32.46% | 18.25% | 18.44% | 16.22% | |||||||
| Russell 1000® Growth Index [Member] | |||||||||||
| Average Annual Return [Line Items] | |||||||||||
| Line Graph and Table Measure Name | Russell 1000® Growth Index | ||||||||||
| Average Annual Return, Percent | 33.36% | 18.96% | 19.14% | 16.78% | |||||||
| Russell Midcap Value Index [Member] | |||||||||||
| Average Annual Return [Line Items] | |||||||||||
| Line Graph and Table Measure Name | Russell Midcap® Value Index | ||||||||||
| Average Annual Return, Percent | 13.07% | 6.65% | |||||||||
| Russell 2500 Index [Member] | |||||||||||
| Average Annual Return [Line Items] | |||||||||||
| Line Graph and Table Measure Name | Russell 2500™ Index | ||||||||||
| Average Annual Return, Percent | 12% | 8.77% | 10.04% | ||||||||
| Russell 2500 Growth Index [Member] | |||||||||||
| Average Annual Return [Line Items] | |||||||||||
| Line Graph and Table Measure Name | Russell 2500™ Growth Index | ||||||||||
| Average Annual Return, Percent | 13.90% | 8.08% | 8.74% | 9.45% | |||||||
| Russell 2500 Value Index [Member] | |||||||||||
| Average Annual Return [Line Items] | |||||||||||
| Line Graph and Table Measure Name | Russell 2500® Value Index | ||||||||||
| Average Annual Return, Percent | 10.98% | 16.09% | |||||||||
| Russell 2000® Growth Index [Member] | |||||||||||
| Average Annual Return [Line Items] | |||||||||||
| Line Graph and Table Measure Name | Russell 2000® Growth Index | ||||||||||
| Average Annual Return, Percent | 15.15% | 6.86% | 7.40% | 8.09% | |||||||
| Russell 2000® Value Index [Member] | |||||||||||
| Average Annual Return [Line Items] | |||||||||||
| Line Graph and Table Measure Name | Russell 2000® Value Index | ||||||||||
| Average Annual Return, Percent | 8.05% | 3.81% | 7.29% | 7.14% | |||||||
| MSCI All Country World ex-U.S. Small Cap Index (net) [Member] | |||||||||||
| Average Annual Return [Line Items] | |||||||||||
| Line Graph and Table Measure Name | MSCI All Country World ex-U.S. Small Cap Index (net) | ||||||||||
| Average Annual Return, Percent | 3.36% | 4.30% | 5.66% | ||||||||
| MSCI Emerging Markets Small Cap Index (net) [Member] | |||||||||||
| Average Annual Return [Line Items] | |||||||||||
| Line Graph and Table Measure Name | MSCI Emerging Markets Small Cap Index (net) | ||||||||||
| Average Annual Return, Percent | 4.79% | 8.56% | 5.73% | ||||||||
| J.P. Morgan Emerging Markets Bond Index (EMBI) Global Diversified Index [Member] | |||||||||||
| Average Annual Return [Line Items] | |||||||||||
| Line Graph and Table Measure Name | J.P. Morgan Emerging Markets Bond Index (EMBI) Global Diversified Index | ||||||||||
| Average Annual Return, Percent | 6.54% | (0.82%) |
| Shareholder Report, Holdings (Details) | Dec. 31, 2024 |
| William Blair Growth Fund - Class I | Amazon.com, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 8.80% |
| William Blair Growth Fund - Class I | Microsoft Corp. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 8.60% |
| William Blair Growth Fund - Class I | Alphabet, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 7.20% |
| William Blair Growth Fund - Class I | Apple, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 4.90% |
| William Blair Growth Fund - Class I | NVIDIA Corp. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 4.80% |
| William Blair Growth Fund - Class I | ServiceNow, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 3.80% |
| William Blair Growth Fund - Class I | Advanced Micro Devices, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 3.50% |
| William Blair Growth Fund - Class I | Mastercard, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 3.50% |
| William Blair Growth Fund - Class I | Texas Instruments, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.80% |
| William Blair Growth Fund - Class I | UnitedHealth Group, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.60% |
| William Blair Growth Fund - Class I | Information Technology Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 43% |
| William Blair Growth Fund - Class I | Consumer Discretionary Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 12% |
| William Blair Growth Fund - Class I | Industrial Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 10.70% |
| William Blair Growth Fund - Class I | Healthcare Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 10.50% |
| William Blair Growth Fund - Class I | Financial Services Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 8.80% |
| William Blair Growth Fund - Class I | Communications Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 7.60% |
| William Blair Growth Fund - Class I | Consumer Staples Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 3.70% |
| William Blair Growth Fund - Class I | Energy Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.90% |
| William Blair Growth Fund - Class N | Amazon.com, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 8.80% |
| William Blair Growth Fund - Class N | Microsoft Corp. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 8.60% |
| William Blair Growth Fund - Class N | Alphabet, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 7.20% |
| William Blair Growth Fund - Class N | Apple, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 4.90% |
| William Blair Growth Fund - Class N | NVIDIA Corp. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 4.80% |
| William Blair Growth Fund - Class N | ServiceNow, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 3.80% |
| William Blair Growth Fund - Class N | Advanced Micro Devices, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 3.50% |
| William Blair Growth Fund - Class N | Mastercard, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 3.50% |
| William Blair Growth Fund - Class N | Texas Instruments, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.80% |
| William Blair Growth Fund - Class N | UnitedHealth Group, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.60% |
| William Blair Growth Fund - Class N | Information Technology Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 43% |
| William Blair Growth Fund - Class N | Consumer Discretionary Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 12% |
| William Blair Growth Fund - Class N | Industrial Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 10.70% |
| William Blair Growth Fund - Class N | Healthcare Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 10.50% |
| William Blair Growth Fund - Class N | Financial Services Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 8.80% |
| William Blair Growth Fund - Class N | Communications Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 7.60% |
| William Blair Growth Fund - Class N | Consumer Staples Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 3.70% |
| William Blair Growth Fund - Class N | Energy Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.90% |
| William Blair Growth Fund - Class R6 | Amazon.com, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 8.80% |
| William Blair Growth Fund - Class R6 | Microsoft Corp. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 8.60% |
| William Blair Growth Fund - Class R6 | Alphabet, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 7.20% |
| William Blair Growth Fund - Class R6 | Apple, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 4.90% |
| William Blair Growth Fund - Class R6 | NVIDIA Corp. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 4.80% |
| William Blair Growth Fund - Class R6 | ServiceNow, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 3.80% |
| William Blair Growth Fund - Class R6 | Advanced Micro Devices, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 3.50% |
| William Blair Growth Fund - Class R6 | Mastercard, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 3.50% |
| William Blair Growth Fund - Class R6 | Texas Instruments, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.80% |
| William Blair Growth Fund - Class R6 | UnitedHealth Group, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.60% |
| William Blair Growth Fund - Class R6 | Information Technology Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 43% |
| William Blair Growth Fund - Class R6 | Consumer Discretionary Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 12% |
| William Blair Growth Fund - Class R6 | Industrial Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 10.70% |
| William Blair Growth Fund - Class R6 | Healthcare Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 10.50% |
| William Blair Growth Fund - Class R6 | Financial Services Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 8.80% |
| William Blair Growth Fund - Class R6 | Communications Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 7.60% |
| William Blair Growth Fund - Class R6 | Consumer Staples Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 3.70% |
| William Blair Growth Fund - Class R6 | Energy Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.90% |
| William Blair Large Cap Growth Fund - Class 1 | Amazon.com, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 9.50% |
| William Blair Large Cap Growth Fund - Class 1 | Microsoft Corp. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 12.20% |
| William Blair Large Cap Growth Fund - Class 1 | Apple, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 9% |
| William Blair Large Cap Growth Fund - Class 1 | NVIDIA Corp. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 11.90% |
| William Blair Large Cap Growth Fund - Class 1 | ServiceNow, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.50% |
| William Blair Large Cap Growth Fund - Class 1 | Mastercard, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 4.70% |
| William Blair Large Cap Growth Fund - Class 1 | UnitedHealth Group, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.40% |
| William Blair Large Cap Growth Fund - Class 1 | Meta Platforms, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 6.50% |
| William Blair Large Cap Growth Fund - Class 1 | Salesforce, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.80% |
| William Blair Large Cap Growth Fund - Class 1 | Intuit, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.30% |
| William Blair Large Cap Growth Fund - Class 1 | Information Technology Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 49.70% |
| William Blair Large Cap Growth Fund - Class 1 | Consumer Discretionary Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 13.70% |
| William Blair Large Cap Growth Fund - Class 1 | Industrial Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 5.70% |
| William Blair Large Cap Growth Fund - Class 1 | Healthcare Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 7.60% |
| William Blair Large Cap Growth Fund - Class 1 | Financial Services Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 8.50% |
| William Blair Large Cap Growth Fund - Class 1 | Communications Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 8.20% |
| William Blair Large Cap Growth Fund - Class 1 | Consumer Staples Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 3.50% |
| William Blair Large Cap Growth Fund - Class 1 | Materials Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.90% |
| William Blair Large Cap Growth Fund - Class N | Amazon.com, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 9.50% |
| William Blair Large Cap Growth Fund - Class N | Microsoft Corp. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 12.20% |
| William Blair Large Cap Growth Fund - Class N | Apple, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 9% |
| William Blair Large Cap Growth Fund - Class N | NVIDIA Corp. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 11.90% |
| William Blair Large Cap Growth Fund - Class N | ServiceNow, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.50% |
| William Blair Large Cap Growth Fund - Class N | Mastercard, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 4.70% |
| William Blair Large Cap Growth Fund - Class N | UnitedHealth Group, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.40% |
| William Blair Large Cap Growth Fund - Class N | Meta Platforms, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 6.50% |
| William Blair Large Cap Growth Fund - Class N | Salesforce, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.80% |
| William Blair Large Cap Growth Fund - Class N | Intuit, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.30% |
| William Blair Large Cap Growth Fund - Class N | Information Technology Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 49.70% |
| William Blair Large Cap Growth Fund - Class N | Consumer Discretionary Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 13.70% |
| William Blair Large Cap Growth Fund - Class N | Industrial Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 5.70% |
| William Blair Large Cap Growth Fund - Class N | Healthcare Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 7.60% |
| William Blair Large Cap Growth Fund - Class N | Financial Services Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 8.50% |
| William Blair Large Cap Growth Fund - Class N | Communications Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 8.20% |
| William Blair Large Cap Growth Fund - Class N | Consumer Staples Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 3.50% |
| William Blair Large Cap Growth Fund - Class N | Materials Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.90% |
| William Blair Large Cap Growth Fund - Class R6 | Amazon.com, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 9.50% |
| William Blair Large Cap Growth Fund - Class R6 | Microsoft Corp. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 12.20% |
| William Blair Large Cap Growth Fund - Class R6 | Apple, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 9% |
| William Blair Large Cap Growth Fund - Class R6 | NVIDIA Corp. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 11.90% |
| William Blair Large Cap Growth Fund - Class R6 | ServiceNow, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.50% |
| William Blair Large Cap Growth Fund - Class R6 | Mastercard, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 4.70% |
| William Blair Large Cap Growth Fund - Class R6 | UnitedHealth Group, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.40% |
| William Blair Large Cap Growth Fund - Class R6 | Meta Platforms, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 6.50% |
| William Blair Large Cap Growth Fund - Class R6 | Salesforce, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.80% |
| William Blair Large Cap Growth Fund - Class R6 | Intuit, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.30% |
| William Blair Large Cap Growth Fund - Class R6 | Information Technology Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 49.70% |
| William Blair Large Cap Growth Fund - Class R6 | Consumer Discretionary Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 13.70% |
| William Blair Large Cap Growth Fund - Class R6 | Industrial Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 5.70% |
| William Blair Large Cap Growth Fund - Class R6 | Healthcare Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 7.60% |
| William Blair Large Cap Growth Fund - Class R6 | Financial Services Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 8.50% |
| William Blair Large Cap Growth Fund - Class R6 | Communications Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 8.20% |
| William Blair Large Cap Growth Fund - Class R6 | Consumer Staples Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 3.50% |
| William Blair Large Cap Growth Fund - Class R6 | Materials Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.90% |
| William Blair Mid Cap Value Fund - Class I | Willis Towers Watson PLC [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.10% |
| William Blair Mid Cap Value Fund - Class I | CRH PLC [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2% |
| William Blair Mid Cap Value Fund - Class I | Regency Centers Corp. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2% |
| William Blair Mid Cap Value Fund - Class I | Genpact Ltd. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.90% |
| William Blair Mid Cap Value Fund - Class I | PPL Corp. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.90% |
| William Blair Mid Cap Value Fund - Class I | U.S. Foods Holding Corp. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.90% |
| William Blair Mid Cap Value Fund - Class I | KKR & Co., Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.90% |
| William Blair Mid Cap Value Fund - Class I | Entergy Corp. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.80% |
| William Blair Mid Cap Value Fund - Class I | Global Payments, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.80% |
| William Blair Mid Cap Value Fund - Class I | NiSource, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.80% |
| William Blair Mid Cap Value Fund - Class I | Information Technology Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 12.80% |
| William Blair Mid Cap Value Fund - Class I | Consumer Discretionary Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 6.60% |
| William Blair Mid Cap Value Fund - Class I | Industrial Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 18.50% |
| William Blair Mid Cap Value Fund - Class I | Healthcare Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 9.70% |
| William Blair Mid Cap Value Fund - Class I | Financial Services Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 17.50% |
| William Blair Mid Cap Value Fund - Class I | Consumer Staples Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 6.70% |
| William Blair Mid Cap Value Fund - Class I | Energy Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 4.30% |
| William Blair Mid Cap Value Fund - Class I | Materials Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 6.30% |
| William Blair Mid Cap Value Fund - Class I | Real Estate Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 8.50% |
| William Blair Mid Cap Value Fund - Class I | Utilities Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 5.60% |
| William Blair Mid Cap Value Fund - Class R6 | Willis Towers Watson PLC [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.10% |
| William Blair Mid Cap Value Fund - Class R6 | CRH PLC [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2% |
| William Blair Mid Cap Value Fund - Class R6 | Regency Centers Corp. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2% |
| William Blair Mid Cap Value Fund - Class R6 | Genpact Ltd. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.90% |
| William Blair Mid Cap Value Fund - Class R6 | PPL Corp. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.90% |
| William Blair Mid Cap Value Fund - Class R6 | U.S. Foods Holding Corp. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.90% |
| William Blair Mid Cap Value Fund - Class R6 | KKR & Co., Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.90% |
| William Blair Mid Cap Value Fund - Class R6 | Entergy Corp. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.80% |
| William Blair Mid Cap Value Fund - Class R6 | Global Payments, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.80% |
| William Blair Mid Cap Value Fund - Class R6 | NiSource, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.80% |
| William Blair Mid Cap Value Fund - Class R6 | Information Technology Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 12.80% |
| William Blair Mid Cap Value Fund - Class R6 | Consumer Discretionary Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 6.60% |
| William Blair Mid Cap Value Fund - Class R6 | Industrial Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 18.50% |
| William Blair Mid Cap Value Fund - Class R6 | Healthcare Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 9.70% |
| William Blair Mid Cap Value Fund - Class R6 | Financial Services Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 17.50% |
| William Blair Mid Cap Value Fund - Class R6 | Consumer Staples Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 6.70% |
| William Blair Mid Cap Value Fund - Class R6 | Energy Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 4.30% |
| William Blair Mid Cap Value Fund - Class R6 | Materials Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 6.30% |
| William Blair Mid Cap Value Fund - Class R6 | Real Estate Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 8.50% |
| William Blair Mid Cap Value Fund - Class R6 | Utilities Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 5.60% |
| William Blair Small-Mid Cap Core Fund - Class I | Carlyle Group, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.50% |
| William Blair Small-Mid Cap Core Fund - Class I | SharkNinja, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.20% |
| William Blair Small-Mid Cap Core Fund - Class I | Western Alliance Bancorp [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.10% |
| William Blair Small-Mid Cap Core Fund - Class I | Primo Brands Corporation [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.90% |
| William Blair Small-Mid Cap Core Fund - Class I | Bright Horizons Family Solutions, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.90% |
| William Blair Small-Mid Cap Core Fund - Class I | TransUnion [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.80% |
| William Blair Small-Mid Cap Core Fund - Class I | Everest Group Ltd. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.80% |
| William Blair Small-Mid Cap Core Fund - Class I | ACV Auctions, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.80% |
| William Blair Small-Mid Cap Core Fund - Class I | Old National Bancorp [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.70% |
| William Blair Small-Mid Cap Core Fund - Class I | Chemed Corp. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.70% |
| William Blair Small-Mid Cap Core Fund - Class I | Information Technology Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 13.10% |
| William Blair Small-Mid Cap Core Fund - Class I | Consumer Discretionary Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 12.70% |
| William Blair Small-Mid Cap Core Fund - Class I | Industrial Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 20.20% |
| William Blair Small-Mid Cap Core Fund - Class I | Healthcare Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 13.50% |
| William Blair Small-Mid Cap Core Fund - Class I | Financial Services Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 15% |
| William Blair Small-Mid Cap Core Fund - Class I | Communications Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 0.50% |
| William Blair Small-Mid Cap Core Fund - Class I | Consumer Staples Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 4.90% |
| William Blair Small-Mid Cap Core Fund - Class I | Energy Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 4.80% |
| William Blair Small-Mid Cap Core Fund - Class I | Materials Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 3.10% |
| William Blair Small-Mid Cap Core Fund - Class I | Real Estate Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 7.40% |
| William Blair Small-Mid Cap Core Fund - Class I | Utilities Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.90% |
| William Blair Small-Mid Cap Core Fund - Class R6 | Carlyle Group, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.50% |
| William Blair Small-Mid Cap Core Fund - Class R6 | SharkNinja, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.20% |
| William Blair Small-Mid Cap Core Fund - Class R6 | Western Alliance Bancorp [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.10% |
| William Blair Small-Mid Cap Core Fund - Class R6 | Primo Brands Corporation [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.90% |
| William Blair Small-Mid Cap Core Fund - Class R6 | Bright Horizons Family Solutions, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.90% |
| William Blair Small-Mid Cap Core Fund - Class R6 | TransUnion [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.80% |
| William Blair Small-Mid Cap Core Fund - Class R6 | Everest Group Ltd. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.80% |
| William Blair Small-Mid Cap Core Fund - Class R6 | ACV Auctions, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.80% |
| William Blair Small-Mid Cap Core Fund - Class R6 | Old National Bancorp [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.70% |
| William Blair Small-Mid Cap Core Fund - Class R6 | Chemed Corp. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.70% |
| William Blair Small-Mid Cap Core Fund - Class R6 | Information Technology Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 13.10% |
| William Blair Small-Mid Cap Core Fund - Class R6 | Consumer Discretionary Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 12.70% |
| William Blair Small-Mid Cap Core Fund - Class R6 | Industrial Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 20.20% |
| William Blair Small-Mid Cap Core Fund - Class R6 | Healthcare Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 13.50% |
| William Blair Small-Mid Cap Core Fund - Class R6 | Financial Services Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 15% |
| William Blair Small-Mid Cap Core Fund - Class R6 | Communications Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 0.50% |
| William Blair Small-Mid Cap Core Fund - Class R6 | Consumer Staples Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 4.90% |
| William Blair Small-Mid Cap Core Fund - Class R6 | Energy Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 4.80% |
| William Blair Small-Mid Cap Core Fund - Class R6 | Materials Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 3.10% |
| William Blair Small-Mid Cap Core Fund - Class R6 | Real Estate Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 7.40% |
| William Blair Small-Mid Cap Core Fund - Class R6 | Utilities Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.90% |
| William Blair Small-Mid Cap Growth Fund - Class I | Carlyle Group, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.30% |
| William Blair Small-Mid Cap Growth Fund - Class I | SharkNinja, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2% |
| William Blair Small-Mid Cap Growth Fund - Class I | Primo Brands Corporation [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.30% |
| William Blair Small-Mid Cap Growth Fund - Class I | Bright Horizons Family Solutions, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.80% |
| William Blair Small-Mid Cap Growth Fund - Class I | Chemed Corp. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.80% |
| William Blair Small-Mid Cap Growth Fund - Class I | e.l.f. Beauty, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.10% |
| William Blair Small-Mid Cap Growth Fund - Class I | Doximity, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.10% |
| William Blair Small-Mid Cap Growth Fund - Class I | Dynatrace, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2% |
| William Blair Small-Mid Cap Growth Fund - Class I | Talen Energy Corporation [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2% |
| William Blair Small-Mid Cap Growth Fund - Class I | Confluent, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2% |
| William Blair Small-Mid Cap Growth Fund - Class I | Information Technology Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 19.30% |
| William Blair Small-Mid Cap Growth Fund - Class I | Consumer Discretionary Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 12% |
| William Blair Small-Mid Cap Growth Fund - Class I | Industrial Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 20.10% |
| William Blair Small-Mid Cap Growth Fund - Class I | Healthcare Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 21.40% |
| William Blair Small-Mid Cap Growth Fund - Class I | Financial Services Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 8.60% |
| William Blair Small-Mid Cap Growth Fund - Class I | Consumer Staples Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 6.70% |
| William Blair Small-Mid Cap Growth Fund - Class I | Energy Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 4.90% |
| William Blair Small-Mid Cap Growth Fund - Class I | Materials Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 3.10% |
| William Blair Small-Mid Cap Growth Fund - Class I | Real Estate Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 0.80% |
| William Blair Small-Mid Cap Growth Fund - Class I | Utilities Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2% |
| William Blair Small-Mid Cap Growth Fund - Class N | Carlyle Group, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.30% |
| William Blair Small-Mid Cap Growth Fund - Class N | SharkNinja, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2% |
| William Blair Small-Mid Cap Growth Fund - Class N | Primo Brands Corporation [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.30% |
| William Blair Small-Mid Cap Growth Fund - Class N | Bright Horizons Family Solutions, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.80% |
| William Blair Small-Mid Cap Growth Fund - Class N | Chemed Corp. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.80% |
| William Blair Small-Mid Cap Growth Fund - Class N | e.l.f. Beauty, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.10% |
| William Blair Small-Mid Cap Growth Fund - Class N | Doximity, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.10% |
| William Blair Small-Mid Cap Growth Fund - Class N | Dynatrace, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2% |
| William Blair Small-Mid Cap Growth Fund - Class N | Talen Energy Corporation [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2% |
| William Blair Small-Mid Cap Growth Fund - Class N | Confluent, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2% |
| William Blair Small-Mid Cap Growth Fund - Class N | Information Technology Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 19.30% |
| William Blair Small-Mid Cap Growth Fund - Class N | Consumer Discretionary Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 12% |
| William Blair Small-Mid Cap Growth Fund - Class N | Industrial Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 20.10% |
| William Blair Small-Mid Cap Growth Fund - Class N | Healthcare Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 21.40% |
| William Blair Small-Mid Cap Growth Fund - Class N | Financial Services Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 8.60% |
| William Blair Small-Mid Cap Growth Fund - Class N | Consumer Staples Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 6.70% |
| William Blair Small-Mid Cap Growth Fund - Class N | Energy Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 4.90% |
| William Blair Small-Mid Cap Growth Fund - Class N | Materials Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 3.10% |
| William Blair Small-Mid Cap Growth Fund - Class N | Real Estate Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 0.80% |
| William Blair Small-Mid Cap Growth Fund - Class N | Utilities Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2% |
| William Blair Small-Mid Cap Growth Fund - Class R6 | Carlyle Group, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.30% |
| William Blair Small-Mid Cap Growth Fund - Class R6 | SharkNinja, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2% |
| William Blair Small-Mid Cap Growth Fund - Class R6 | Primo Brands Corporation [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.30% |
| William Blair Small-Mid Cap Growth Fund - Class R6 | Bright Horizons Family Solutions, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.80% |
| William Blair Small-Mid Cap Growth Fund - Class R6 | Chemed Corp. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.80% |
| William Blair Small-Mid Cap Growth Fund - Class R6 | e.l.f. Beauty, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.10% |
| William Blair Small-Mid Cap Growth Fund - Class R6 | Doximity, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.10% |
| William Blair Small-Mid Cap Growth Fund - Class R6 | Dynatrace, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2% |
| William Blair Small-Mid Cap Growth Fund - Class R6 | Talen Energy Corporation [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2% |
| William Blair Small-Mid Cap Growth Fund - Class R6 | Confluent, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2% |
| William Blair Small-Mid Cap Growth Fund - Class R6 | Information Technology Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 19.30% |
| William Blair Small-Mid Cap Growth Fund - Class R6 | Consumer Discretionary Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 12% |
| William Blair Small-Mid Cap Growth Fund - Class R6 | Industrial Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 20.10% |
| William Blair Small-Mid Cap Growth Fund - Class R6 | Healthcare Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 21.40% |
| William Blair Small-Mid Cap Growth Fund - Class R6 | Financial Services Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 8.60% |
| William Blair Small-Mid Cap Growth Fund - Class R6 | Consumer Staples Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 6.70% |
| William Blair Small-Mid Cap Growth Fund - Class R6 | Energy Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 4.90% |
| William Blair Small-Mid Cap Growth Fund - Class R6 | Materials Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 3.10% |
| William Blair Small-Mid Cap Growth Fund - Class R6 | Real Estate Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 0.80% |
| William Blair Small-Mid Cap Growth Fund - Class R6 | Utilities Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2% |
| William Blair Small-Mid Cap Value Fund - Class I | Genpact Ltd. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.40% |
| William Blair Small-Mid Cap Value Fund - Class I | U.S. Foods Holding Corp. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.40% |
| William Blair Small-Mid Cap Value Fund - Class I | Carlyle Group, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.40% |
| William Blair Small-Mid Cap Value Fund - Class I | Columbia Banking System, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.40% |
| William Blair Small-Mid Cap Value Fund - Class I | Silgan Holdings, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.40% |
| William Blair Small-Mid Cap Value Fund - Class I | Labcorp Holdings, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.40% |
| William Blair Small-Mid Cap Value Fund - Class I | Tecnoglass, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.40% |
| William Blair Small-Mid Cap Value Fund - Class I | Air Lease Corp. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.40% |
| William Blair Small-Mid Cap Value Fund - Class I | Avnet, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.40% |
| William Blair Small-Mid Cap Value Fund - Class I | Maximus, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.40% |
| William Blair Small-Mid Cap Value Fund - Class I | Information Technology Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 9.10% |
| William Blair Small-Mid Cap Value Fund - Class I | Consumer Discretionary Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 7.20% |
| William Blair Small-Mid Cap Value Fund - Class I | Industrial Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 26.10% |
| William Blair Small-Mid Cap Value Fund - Class I | Healthcare Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 8.60% |
| William Blair Small-Mid Cap Value Fund - Class I | Financial Services Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 16.60% |
| William Blair Small-Mid Cap Value Fund - Class I | Communications Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.20% |
| William Blair Small-Mid Cap Value Fund - Class I | Consumer Staples Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 4.80% |
| William Blair Small-Mid Cap Value Fund - Class I | Energy Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 5.30% |
| William Blair Small-Mid Cap Value Fund - Class I | Materials Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 6.70% |
| William Blair Small-Mid Cap Value Fund - Class I | Real Estate Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 8.80% |
| William Blair Small-Mid Cap Value Fund - Class I | Utilities Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 3.50% |
| William Blair Small-Mid Cap Value Fund - Class R6 | Genpact Ltd. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.40% |
| William Blair Small-Mid Cap Value Fund - Class R6 | U.S. Foods Holding Corp. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.40% |
| William Blair Small-Mid Cap Value Fund - Class R6 | Carlyle Group, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.40% |
| William Blair Small-Mid Cap Value Fund - Class R6 | Columbia Banking System, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.40% |
| William Blair Small-Mid Cap Value Fund - Class R6 | Silgan Holdings, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.40% |
| William Blair Small-Mid Cap Value Fund - Class R6 | Labcorp Holdings, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.40% |
| William Blair Small-Mid Cap Value Fund - Class R6 | Tecnoglass, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.40% |
| William Blair Small-Mid Cap Value Fund - Class R6 | Air Lease Corp. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.40% |
| William Blair Small-Mid Cap Value Fund - Class R6 | Avnet, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.40% |
| William Blair Small-Mid Cap Value Fund - Class R6 | Maximus, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.40% |
| William Blair Small-Mid Cap Value Fund - Class R6 | Information Technology Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 9.10% |
| William Blair Small-Mid Cap Value Fund - Class R6 | Consumer Discretionary Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 7.20% |
| William Blair Small-Mid Cap Value Fund - Class R6 | Industrial Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 26.10% |
| William Blair Small-Mid Cap Value Fund - Class R6 | Healthcare Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 8.60% |
| William Blair Small-Mid Cap Value Fund - Class R6 | Financial Services Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 16.60% |
| William Blair Small-Mid Cap Value Fund - Class R6 | Communications Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.20% |
| William Blair Small-Mid Cap Value Fund - Class R6 | Consumer Staples Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 4.80% |
| William Blair Small-Mid Cap Value Fund - Class R6 | Energy Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 5.30% |
| William Blair Small-Mid Cap Value Fund - Class R6 | Materials Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 6.70% |
| William Blair Small-Mid Cap Value Fund - Class R6 | Real Estate Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 8.80% |
| William Blair Small-Mid Cap Value Fund - Class R6 | Utilities Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 3.50% |
| William Blair Small Cap Growth Fund - Class 1 | ACV Auctions, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.90% |
| William Blair Small Cap Growth Fund - Class 1 | Doximity, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.90% |
| William Blair Small Cap Growth Fund - Class 1 | Casella Waste Systems, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.60% |
| William Blair Small Cap Growth Fund - Class 1 | Mercury Systems, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.70% |
| William Blair Small Cap Growth Fund - Class 1 | Brink's Co. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.70% |
| William Blair Small Cap Growth Fund - Class 1 | Vericel Corp. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.70% |
| William Blair Small Cap Growth Fund - Class 1 | FirstService Corp. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.70% |
| William Blair Small Cap Growth Fund - Class 1 | Balchem Corp. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.60% |
| William Blair Small Cap Growth Fund - Class 1 | FTAI Aviation Ltd. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.60% |
| William Blair Small Cap Growth Fund - Class 1 | Planet Fitness, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.50% |
| William Blair Small Cap Growth Fund - Class 1 | Information Technology Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 19.10% |
| William Blair Small Cap Growth Fund - Class 1 | Consumer Discretionary Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 12.30% |
| William Blair Small Cap Growth Fund - Class 1 | Industrial Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 18.70% |
| William Blair Small Cap Growth Fund - Class 1 | Healthcare Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 23.60% |
| William Blair Small Cap Growth Fund - Class 1 | Financial Services Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 8.70% |
| William Blair Small Cap Growth Fund - Class 1 | Communications Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.30% |
| William Blair Small Cap Growth Fund - Class 1 | Consumer Staples Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 4.20% |
| William Blair Small Cap Growth Fund - Class 1 | Energy Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 3.80% |
| William Blair Small Cap Growth Fund - Class 1 | Materials Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 4.20% |
| William Blair Small Cap Growth Fund - Class 1 | Real Estate Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.70% |
| William Blair Small Cap Growth Fund - Class 1 | Exchange-Traded Funds [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1% |
| William Blair Small Cap Growth Fund - Class N | ACV Auctions, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.90% |
| William Blair Small Cap Growth Fund - Class N | Doximity, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.90% |
| William Blair Small Cap Growth Fund - Class N | Casella Waste Systems, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.60% |
| William Blair Small Cap Growth Fund - Class N | Mercury Systems, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.70% |
| William Blair Small Cap Growth Fund - Class N | Brink's Co. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.70% |
| William Blair Small Cap Growth Fund - Class N | Vericel Corp. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.70% |
| William Blair Small Cap Growth Fund - Class N | FirstService Corp. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.70% |
| William Blair Small Cap Growth Fund - Class N | Balchem Corp. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.60% |
| William Blair Small Cap Growth Fund - Class N | FTAI Aviation Ltd. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.60% |
| William Blair Small Cap Growth Fund - Class N | Planet Fitness, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.50% |
| William Blair Small Cap Growth Fund - Class N | Information Technology Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 19.10% |
| William Blair Small Cap Growth Fund - Class N | Consumer Discretionary Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 12.30% |
| William Blair Small Cap Growth Fund - Class N | Industrial Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 18.70% |
| William Blair Small Cap Growth Fund - Class N | Healthcare Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 23.60% |
| William Blair Small Cap Growth Fund - Class N | Financial Services Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 8.70% |
| William Blair Small Cap Growth Fund - Class N | Communications Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.30% |
| William Blair Small Cap Growth Fund - Class N | Consumer Staples Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 4.20% |
| William Blair Small Cap Growth Fund - Class N | Energy Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 3.80% |
| William Blair Small Cap Growth Fund - Class N | Materials Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 4.20% |
| William Blair Small Cap Growth Fund - Class N | Real Estate Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.70% |
| William Blair Small Cap Growth Fund - Class N | Exchange-Traded Funds [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1% |
| William Blair Small Cap Growth Fund - Class R6 | ACV Auctions, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.90% |
| William Blair Small Cap Growth Fund - Class R6 | Doximity, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.90% |
| William Blair Small Cap Growth Fund - Class R6 | Casella Waste Systems, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.60% |
| William Blair Small Cap Growth Fund - Class R6 | Mercury Systems, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.70% |
| William Blair Small Cap Growth Fund - Class R6 | Brink's Co. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.70% |
| William Blair Small Cap Growth Fund - Class R6 | Vericel Corp. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.70% |
| William Blair Small Cap Growth Fund - Class R6 | FirstService Corp. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.70% |
| William Blair Small Cap Growth Fund - Class R6 | Balchem Corp. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.60% |
| William Blair Small Cap Growth Fund - Class R6 | FTAI Aviation Ltd. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.60% |
| William Blair Small Cap Growth Fund - Class R6 | Planet Fitness, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.50% |
| William Blair Small Cap Growth Fund - Class R6 | Information Technology Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 19.10% |
| William Blair Small Cap Growth Fund - Class R6 | Consumer Discretionary Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 12.30% |
| William Blair Small Cap Growth Fund - Class R6 | Industrial Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 18.70% |
| William Blair Small Cap Growth Fund - Class R6 | Healthcare Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 23.60% |
| William Blair Small Cap Growth Fund - Class R6 | Financial Services Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 8.70% |
| William Blair Small Cap Growth Fund - Class R6 | Communications Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.30% |
| William Blair Small Cap Growth Fund - Class R6 | Consumer Staples Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 4.20% |
| William Blair Small Cap Growth Fund - Class R6 | Energy Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 3.80% |
| William Blair Small Cap Growth Fund - Class R6 | Materials Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 4.20% |
| William Blair Small Cap Growth Fund - Class R6 | Real Estate Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.70% |
| William Blair Small Cap Growth Fund - Class R6 | Exchange-Traded Funds [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1% |
| William Blair Small Cap value fund - class I | Old National Bancorp [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.30% |
| William Blair Small Cap value fund - class I | Columbia Banking System, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.40% |
| William Blair Small Cap value fund - class I | Tower Semiconductor Ltd. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.50% |
| William Blair Small Cap value fund - class I | Ameris Bancorp [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.40% |
| William Blair Small Cap value fund - class I | Texas Capital Bancshares, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.40% |
| William Blair Small Cap value fund - class I | Taylor Morrison Home Corp. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.40% |
| William Blair Small Cap value fund - class I | Gulfport Energy Corp. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.30% |
| William Blair Small Cap value fund - class I | John Wiley & Sons, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.30% |
| William Blair Small Cap value fund - class I | Knowles Corp. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.30% |
| William Blair Small Cap value fund - class I | La-Z-Boy, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.20% |
| William Blair Small Cap value fund - class I | Information Technology Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 11.30% |
| William Blair Small Cap value fund - class I | Consumer Discretionary Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 12.30% |
| William Blair Small Cap value fund - class I | Industrial Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 22.40% |
| William Blair Small Cap value fund - class I | Healthcare Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 3.40% |
| William Blair Small Cap value fund - class I | Financial Services Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 23.90% |
| William Blair Small Cap value fund - class I | Communications Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.30% |
| William Blair Small Cap value fund - class I | Consumer Staples Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.50% |
| William Blair Small Cap value fund - class I | Energy Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 5.10% |
| William Blair Small Cap value fund - class I | Materials Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 4.40% |
| William Blair Small Cap value fund - class I | Real Estate Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 9.80% |
| William Blair Small Cap value fund - class I | Utilities Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.70% |
| William Blair Small Cap value fund - class N | Old National Bancorp [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.30% |
| William Blair Small Cap value fund - class N | Columbia Banking System, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.40% |
| William Blair Small Cap value fund - class N | Tower Semiconductor Ltd. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.50% |
| William Blair Small Cap value fund - class N | Ameris Bancorp [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.40% |
| William Blair Small Cap value fund - class N | Texas Capital Bancshares, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.40% |
| William Blair Small Cap value fund - class N | Taylor Morrison Home Corp. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.40% |
| William Blair Small Cap value fund - class N | Gulfport Energy Corp. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.30% |
| William Blair Small Cap value fund - class N | John Wiley & Sons, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.30% |
| William Blair Small Cap value fund - class N | Knowles Corp. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.30% |
| William Blair Small Cap value fund - class N | La-Z-Boy, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.20% |
| William Blair Small Cap value fund - class N | Information Technology Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 11.30% |
| William Blair Small Cap value fund - class N | Consumer Discretionary Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 12.30% |
| William Blair Small Cap value fund - class N | Industrial Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 22.40% |
| William Blair Small Cap value fund - class N | Healthcare Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 3.40% |
| William Blair Small Cap value fund - class N | Financial Services Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 23.90% |
| William Blair Small Cap value fund - class N | Communications Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.30% |
| William Blair Small Cap value fund - class N | Consumer Staples Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.50% |
| William Blair Small Cap value fund - class N | Energy Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 5.10% |
| William Blair Small Cap value fund - class N | Materials Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 4.40% |
| William Blair Small Cap value fund - class N | Real Estate Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 9.80% |
| William Blair Small Cap value fund - class N | Utilities Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.70% |
| William Blair Small Cap Value Fund - Class R6 [Member] | Old National Bancorp [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.30% |
| William Blair Small Cap Value Fund - Class R6 [Member] | Columbia Banking System, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.40% |
| William Blair Small Cap Value Fund - Class R6 [Member] | Tower Semiconductor Ltd. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.50% |
| William Blair Small Cap Value Fund - Class R6 [Member] | Ameris Bancorp [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.40% |
| William Blair Small Cap Value Fund - Class R6 [Member] | Texas Capital Bancshares, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.40% |
| William Blair Small Cap Value Fund - Class R6 [Member] | Taylor Morrison Home Corp. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.40% |
| William Blair Small Cap Value Fund - Class R6 [Member] | Gulfport Energy Corp. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.30% |
| William Blair Small Cap Value Fund - Class R6 [Member] | John Wiley & Sons, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.30% |
| William Blair Small Cap Value Fund - Class R6 [Member] | Knowles Corp. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.30% |
| William Blair Small Cap Value Fund - Class R6 [Member] | La-Z-Boy, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.20% |
| William Blair Small Cap Value Fund - Class R6 [Member] | Information Technology Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 11.30% |
| William Blair Small Cap Value Fund - Class R6 [Member] | Consumer Discretionary Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 12.30% |
| William Blair Small Cap Value Fund - Class R6 [Member] | Industrial Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 22.40% |
| William Blair Small Cap Value Fund - Class R6 [Member] | Healthcare Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 3.40% |
| William Blair Small Cap Value Fund - Class R6 [Member] | Financial Services Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 23.90% |
| William Blair Small Cap Value Fund - Class R6 [Member] | Communications Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.30% |
| William Blair Small Cap Value Fund - Class R6 [Member] | Consumer Staples Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.50% |
| William Blair Small Cap Value Fund - Class R6 [Member] | Energy Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 5.10% |
| William Blair Small Cap Value Fund - Class R6 [Member] | Materials Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 4.40% |
| William Blair Small Cap Value Fund - Class R6 [Member] | Real Estate Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 9.80% |
| William Blair Small Cap Value Fund - Class R6 [Member] | Utilities Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.70% |
| William Blair Global Leaders Fund - Class I | Amazon.com, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 4.70% |
| William Blair Global Leaders Fund - Class I | Microsoft Corp. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 5.70% |
| William Blair Global Leaders Fund - Class I | Alphabet, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 3.90% |
| William Blair Global Leaders Fund - Class I | NVIDIA Corp. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 4.40% |
| William Blair Global Leaders Fund - Class I | Mastercard, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 3.20% |
| William Blair Global Leaders Fund - Class I | Meta Platforms, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 3% |
| William Blair Global Leaders Fund - Class I | Taiwan Semiconductor Manufacturing Co. Ltd. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 3.30% |
| William Blair Global Leaders Fund - Class I | Compass Group PLC [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.10% |
| William Blair Global Leaders Fund - Class I | Hermes International SCA [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.10% |
| William Blair Global Leaders Fund - Class I | Broadcom, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.10% |
| William Blair Global Leaders Fund - Class I | Information Technology Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 28.30% |
| William Blair Global Leaders Fund - Class I | Consumer Discretionary Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 16.40% |
| William Blair Global Leaders Fund - Class I | Industrial Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 18.80% |
| William Blair Global Leaders Fund - Class I | Healthcare Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 7.60% |
| William Blair Global Leaders Fund - Class I | Financial Services Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 14.10% |
| William Blair Global Leaders Fund - Class I | Communications Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 6.90% |
| William Blair Global Leaders Fund - Class I | Consumer Staples Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.40% |
| William Blair Global Leaders Fund - Class I | Energy Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.70% |
| William Blair Global Leaders Fund - Class I | Materials Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 3.40% |
| William Blair Global Leaders Fund - Class I | Utilities Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 0.90% |
| William Blair Global Leaders Fund - Class I | UNITED KINGDOM | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 8.10% |
| William Blair Global Leaders Fund - Class I | JAPAN | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 0.70% |
| William Blair Global Leaders Fund - Class I | Western Hemisphere [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 64.30% |
| William Blair Global Leaders Fund - Class I | Europe Ex-U.K. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 20.70% |
| William Blair Global Leaders Fund - Class I | Emerging Asia [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 5.70% |
| William Blair Global Leaders Fund - Class N | Amazon.com, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 4.70% |
| William Blair Global Leaders Fund - Class N | Microsoft Corp. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 5.70% |
| William Blair Global Leaders Fund - Class N | Alphabet, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 3.90% |
| William Blair Global Leaders Fund - Class N | NVIDIA Corp. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 4.40% |
| William Blair Global Leaders Fund - Class N | Mastercard, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 3.20% |
| William Blair Global Leaders Fund - Class N | Meta Platforms, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 3% |
| William Blair Global Leaders Fund - Class N | Taiwan Semiconductor Manufacturing Co. Ltd. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 3.30% |
| William Blair Global Leaders Fund - Class N | Compass Group PLC [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.10% |
| William Blair Global Leaders Fund - Class N | Hermes International SCA [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.10% |
| William Blair Global Leaders Fund - Class N | Broadcom, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.10% |
| William Blair Global Leaders Fund - Class N | Information Technology Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 28.30% |
| William Blair Global Leaders Fund - Class N | Consumer Discretionary Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 16.40% |
| William Blair Global Leaders Fund - Class N | Industrial Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 18.80% |
| William Blair Global Leaders Fund - Class N | Healthcare Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 7.60% |
| William Blair Global Leaders Fund - Class N | Financial Services Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 14.10% |
| William Blair Global Leaders Fund - Class N | Communications Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 6.90% |
| William Blair Global Leaders Fund - Class N | Consumer Staples Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.40% |
| William Blair Global Leaders Fund - Class N | Energy Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.70% |
| William Blair Global Leaders Fund - Class N | Materials Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 3.40% |
| William Blair Global Leaders Fund - Class N | Utilities Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 0.90% |
| William Blair Global Leaders Fund - Class N | UNITED KINGDOM | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 8.10% |
| William Blair Global Leaders Fund - Class N | JAPAN | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 0.70% |
| William Blair Global Leaders Fund - Class N | Western Hemisphere [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 64.30% |
| William Blair Global Leaders Fund - Class N | Europe Ex-U.K. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 20.70% |
| William Blair Global Leaders Fund - Class N | Emerging Asia [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 5.70% |
| William Blair Global Leaders Fund - Class R6 | Amazon.com, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 4.70% |
| William Blair Global Leaders Fund - Class R6 | Microsoft Corp. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 5.70% |
| William Blair Global Leaders Fund - Class R6 | Alphabet, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 3.90% |
| William Blair Global Leaders Fund - Class R6 | NVIDIA Corp. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 4.40% |
| William Blair Global Leaders Fund - Class R6 | Mastercard, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 3.20% |
| William Blair Global Leaders Fund - Class R6 | Meta Platforms, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 3% |
| William Blair Global Leaders Fund - Class R6 | Taiwan Semiconductor Manufacturing Co. Ltd. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 3.30% |
| William Blair Global Leaders Fund - Class R6 | Compass Group PLC [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.10% |
| William Blair Global Leaders Fund - Class R6 | Hermes International SCA [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.10% |
| William Blair Global Leaders Fund - Class R6 | Broadcom, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.10% |
| William Blair Global Leaders Fund - Class R6 | Information Technology Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 28.30% |
| William Blair Global Leaders Fund - Class R6 | Consumer Discretionary Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 16.40% |
| William Blair Global Leaders Fund - Class R6 | Industrial Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 18.80% |
| William Blair Global Leaders Fund - Class R6 | Healthcare Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 7.60% |
| William Blair Global Leaders Fund - Class R6 | Financial Services Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 14.10% |
| William Blair Global Leaders Fund - Class R6 | Communications Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 6.90% |
| William Blair Global Leaders Fund - Class R6 | Consumer Staples Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.40% |
| William Blair Global Leaders Fund - Class R6 | Energy Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.70% |
| William Blair Global Leaders Fund - Class R6 | Materials Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 3.40% |
| William Blair Global Leaders Fund - Class R6 | Utilities Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 0.90% |
| William Blair Global Leaders Fund - Class R6 | UNITED KINGDOM | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 8.10% |
| William Blair Global Leaders Fund - Class R6 | JAPAN | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 0.70% |
| William Blair Global Leaders Fund - Class R6 | Western Hemisphere [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 64.30% |
| William Blair Global Leaders Fund - Class R6 | Europe Ex-U.K. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 20.70% |
| William Blair Global Leaders Fund - Class R6 | Emerging Asia [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 5.70% |
| William Blair International Leaders Fund - Class I | Taiwan Semiconductor Manufacturing Co. Ltd. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 4.30% |
| William Blair International Leaders Fund - Class I | Compass Group PLC [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2% |
| William Blair International Leaders Fund - Class I | SAP SE [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.60% |
| William Blair International Leaders Fund - Class I | MediaTek, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.30% |
| William Blair International Leaders Fund - Class I | Schneider Electric SE [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.20% |
| William Blair International Leaders Fund - Class I | Industria de Diseno Textil SA [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.20% |
| William Blair International Leaders Fund - Class I | London Stock Exchange Group PLC [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.10% |
| William Blair International Leaders Fund - Class I | Constellation Software, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.10% |
| William Blair International Leaders Fund - Class I | Sandoz Group AG [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2% |
| William Blair International Leaders Fund - Class I | Intact Financial Corp. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2% |
| William Blair International Leaders Fund - Class I | Information Technology Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 21.20% |
| William Blair International Leaders Fund - Class I | Consumer Discretionary Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 18.10% |
| William Blair International Leaders Fund - Class I | Industrial Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 20.50% |
| William Blair International Leaders Fund - Class I | Healthcare Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 12% |
| William Blair International Leaders Fund - Class I | Financial Services Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 16.50% |
| William Blair International Leaders Fund - Class I | Communications Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.70% |
| William Blair International Leaders Fund - Class I | Consumer Staples Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.30% |
| William Blair International Leaders Fund - Class I | Energy Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.70% |
| William Blair International Leaders Fund - Class I | Materials Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 4.70% |
| William Blair International Leaders Fund - Class I | UNITED KINGDOM | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 14.70% |
| William Blair International Leaders Fund - Class I | JAPAN | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 14.90% |
| William Blair International Leaders Fund - Class I | Western Hemisphere [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 8.80% |
| William Blair International Leaders Fund - Class I | Europe Ex-U.K. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 38.90% |
| William Blair International Leaders Fund - Class I | Emerging Asia [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 16% |
| William Blair International Leaders Fund - Class I | Asia Ex-Japan [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 3.90% |
| William Blair International Leaders Fund - Class I | Emerging Latin America [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.50% |
| William Blair International Leaders Fund - Class N | Taiwan Semiconductor Manufacturing Co. Ltd. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 4.30% |
| William Blair International Leaders Fund - Class N | Compass Group PLC [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2% |
| William Blair International Leaders Fund - Class N | SAP SE [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.60% |
| William Blair International Leaders Fund - Class N | MediaTek, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.30% |
| William Blair International Leaders Fund - Class N | Schneider Electric SE [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.20% |
| William Blair International Leaders Fund - Class N | Industria de Diseno Textil SA [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.20% |
| William Blair International Leaders Fund - Class N | London Stock Exchange Group PLC [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.10% |
| William Blair International Leaders Fund - Class N | Constellation Software, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.10% |
| William Blair International Leaders Fund - Class N | Sandoz Group AG [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2% |
| William Blair International Leaders Fund - Class N | Intact Financial Corp. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2% |
| William Blair International Leaders Fund - Class N | Information Technology Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 21.20% |
| William Blair International Leaders Fund - Class N | Consumer Discretionary Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 18.10% |
| William Blair International Leaders Fund - Class N | Industrial Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 20.50% |
| William Blair International Leaders Fund - Class N | Healthcare Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 12% |
| William Blair International Leaders Fund - Class N | Financial Services Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 16.50% |
| William Blair International Leaders Fund - Class N | Communications Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.70% |
| William Blair International Leaders Fund - Class N | Consumer Staples Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.30% |
| William Blair International Leaders Fund - Class N | Energy Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.70% |
| William Blair International Leaders Fund - Class N | Materials Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 4.70% |
| William Blair International Leaders Fund - Class N | UNITED KINGDOM | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 14.70% |
| William Blair International Leaders Fund - Class N | JAPAN | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 14.90% |
| William Blair International Leaders Fund - Class N | Western Hemisphere [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 8.80% |
| William Blair International Leaders Fund - Class N | Europe Ex-U.K. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 38.90% |
| William Blair International Leaders Fund - Class N | Emerging Asia [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 16% |
| William Blair International Leaders Fund - Class N | Asia Ex-Japan [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 3.90% |
| William Blair International Leaders Fund - Class N | Emerging Latin America [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.50% |
| William Blair International Leaders Fund Class R6 | Taiwan Semiconductor Manufacturing Co. Ltd. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 4.30% |
| William Blair International Leaders Fund Class R6 | Compass Group PLC [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2% |
| William Blair International Leaders Fund Class R6 | SAP SE [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.60% |
| William Blair International Leaders Fund Class R6 | MediaTek, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.30% |
| William Blair International Leaders Fund Class R6 | Schneider Electric SE [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.20% |
| William Blair International Leaders Fund Class R6 | Industria de Diseno Textil SA [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.20% |
| William Blair International Leaders Fund Class R6 | London Stock Exchange Group PLC [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.10% |
| William Blair International Leaders Fund Class R6 | Constellation Software, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.10% |
| William Blair International Leaders Fund Class R6 | Sandoz Group AG [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2% |
| William Blair International Leaders Fund Class R6 | Intact Financial Corp. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2% |
| William Blair International Leaders Fund Class R6 | Information Technology Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 21.20% |
| William Blair International Leaders Fund Class R6 | Consumer Discretionary Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 18.10% |
| William Blair International Leaders Fund Class R6 | Industrial Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 20.50% |
| William Blair International Leaders Fund Class R6 | Healthcare Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 12% |
| William Blair International Leaders Fund Class R6 | Financial Services Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 16.50% |
| William Blair International Leaders Fund Class R6 | Communications Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.70% |
| William Blair International Leaders Fund Class R6 | Consumer Staples Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.30% |
| William Blair International Leaders Fund Class R6 | Energy Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.70% |
| William Blair International Leaders Fund Class R6 | Materials Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 4.70% |
| William Blair International Leaders Fund Class R6 | UNITED KINGDOM | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 14.70% |
| William Blair International Leaders Fund Class R6 | JAPAN | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 14.90% |
| William Blair International Leaders Fund Class R6 | Western Hemisphere [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 8.80% |
| William Blair International Leaders Fund Class R6 | Europe Ex-U.K. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 38.90% |
| William Blair International Leaders Fund Class R6 | Emerging Asia [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 16% |
| William Blair International Leaders Fund Class R6 | Asia Ex-Japan [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 3.90% |
| William Blair International Leaders Fund Class R6 | Emerging Latin America [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.50% |
| William Blair International Growth Fund - Class I | Taiwan Semiconductor Manufacturing Co. Ltd. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 3.90% |
| William Blair International Growth Fund - Class I | Hermes International SCA [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.30% |
| William Blair International Growth Fund - Class I | SAP SE [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.70% |
| William Blair International Growth Fund - Class I | London Stock Exchange Group PLC [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.30% |
| William Blair International Growth Fund - Class I | Tencent Holdings Ltd. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2% |
| William Blair International Growth Fund - Class I | Threei Group PLC [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.60% |
| William Blair International Growth Fund - Class I | Lonza Group AG [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.20% |
| William Blair International Growth Fund - Class I | DSV AS [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.20% |
| William Blair International Growth Fund - Class I | Bank Central Asia Tbk. PT [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.20% |
| William Blair International Growth Fund - Class I | MTU Aero Engines AG [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.20% |
| William Blair International Growth Fund - Class I | Information Technology Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 18.40% |
| William Blair International Growth Fund - Class I | Consumer Discretionary Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 14.80% |
| William Blair International Growth Fund - Class I | Industrial Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 23.80% |
| William Blair International Growth Fund - Class I | Healthcare Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 10.40% |
| William Blair International Growth Fund - Class I | Financial Services Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 17% |
| William Blair International Growth Fund - Class I | Communications Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 4.40% |
| William Blair International Growth Fund - Class I | Consumer Staples Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 3.60% |
| William Blair International Growth Fund - Class I | Energy Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.30% |
| William Blair International Growth Fund - Class I | Materials Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2% |
| William Blair International Growth Fund - Class I | Real Estate Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 0.80% |
| William Blair International Growth Fund - Class I | UNITED KINGDOM | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 12.90% |
| William Blair International Growth Fund - Class I | JAPAN | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 12.90% |
| William Blair International Growth Fund - Class I | Western Hemisphere [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 8.20% |
| William Blair International Growth Fund - Class I | Europe Ex-U.K. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 35.70% |
| William Blair International Growth Fund - Class I | Emerging Asia [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 19.60% |
| William Blair International Growth Fund - Class I | Asia Ex-Japan [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 4.70% |
| William Blair International Growth Fund - Class I | Emerging Latin America [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.40% |
| William Blair International Growth Fund - Class I | Emerging Europe, Mid-East, Africa [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.10% |
| William Blair International Growth Fund - Class N | Taiwan Semiconductor Manufacturing Co. Ltd. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 3.90% |
| William Blair International Growth Fund - Class N | Hermes International SCA [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.30% |
| William Blair International Growth Fund - Class N | SAP SE [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.70% |
| William Blair International Growth Fund - Class N | London Stock Exchange Group PLC [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.30% |
| William Blair International Growth Fund - Class N | Tencent Holdings Ltd. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2% |
| William Blair International Growth Fund - Class N | Threei Group PLC [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.60% |
| William Blair International Growth Fund - Class N | Lonza Group AG [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.20% |
| William Blair International Growth Fund - Class N | DSV AS [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.20% |
| William Blair International Growth Fund - Class N | Bank Central Asia Tbk. PT [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.20% |
| William Blair International Growth Fund - Class N | MTU Aero Engines AG [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.20% |
| William Blair International Growth Fund - Class N | Information Technology Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 18.40% |
| William Blair International Growth Fund - Class N | Consumer Discretionary Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 14.80% |
| William Blair International Growth Fund - Class N | Industrial Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 23.80% |
| William Blair International Growth Fund - Class N | Healthcare Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 10.40% |
| William Blair International Growth Fund - Class N | Financial Services Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 17% |
| William Blair International Growth Fund - Class N | Communications Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 4.40% |
| William Blair International Growth Fund - Class N | Consumer Staples Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 3.60% |
| William Blair International Growth Fund - Class N | Energy Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.30% |
| William Blair International Growth Fund - Class N | Materials Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2% |
| William Blair International Growth Fund - Class N | Real Estate Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 0.80% |
| William Blair International Growth Fund - Class N | UNITED KINGDOM | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 12.90% |
| William Blair International Growth Fund - Class N | JAPAN | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 12.90% |
| William Blair International Growth Fund - Class N | Western Hemisphere [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 8.20% |
| William Blair International Growth Fund - Class N | Europe Ex-U.K. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 35.70% |
| William Blair International Growth Fund - Class N | Emerging Asia [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 19.60% |
| William Blair International Growth Fund - Class N | Asia Ex-Japan [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 4.70% |
| William Blair International Growth Fund - Class N | Emerging Latin America [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.40% |
| William Blair International Growth Fund - Class N | Emerging Europe, Mid-East, Africa [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.10% |
| William Blair International Growth Fund - Class R6 | Taiwan Semiconductor Manufacturing Co. Ltd. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 3.90% |
| William Blair International Growth Fund - Class R6 | Hermes International SCA [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.30% |
| William Blair International Growth Fund - Class R6 | SAP SE [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.70% |
| William Blair International Growth Fund - Class R6 | London Stock Exchange Group PLC [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.30% |
| William Blair International Growth Fund - Class R6 | Tencent Holdings Ltd. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2% |
| William Blair International Growth Fund - Class R6 | Threei Group PLC [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.60% |
| William Blair International Growth Fund - Class R6 | Lonza Group AG [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.20% |
| William Blair International Growth Fund - Class R6 | DSV AS [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.20% |
| William Blair International Growth Fund - Class R6 | Bank Central Asia Tbk. PT [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.20% |
| William Blair International Growth Fund - Class R6 | MTU Aero Engines AG [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.20% |
| William Blair International Growth Fund - Class R6 | Information Technology Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 18.40% |
| William Blair International Growth Fund - Class R6 | Consumer Discretionary Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 14.80% |
| William Blair International Growth Fund - Class R6 | Industrial Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 23.80% |
| William Blair International Growth Fund - Class R6 | Healthcare Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 10.40% |
| William Blair International Growth Fund - Class R6 | Financial Services Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 17% |
| William Blair International Growth Fund - Class R6 | Communications Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 4.40% |
| William Blair International Growth Fund - Class R6 | Consumer Staples Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 3.60% |
| William Blair International Growth Fund - Class R6 | Energy Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.30% |
| William Blair International Growth Fund - Class R6 | Materials Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2% |
| William Blair International Growth Fund - Class R6 | Real Estate Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 0.80% |
| William Blair International Growth Fund - Class R6 | UNITED KINGDOM | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 12.90% |
| William Blair International Growth Fund - Class R6 | JAPAN | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 12.90% |
| William Blair International Growth Fund - Class R6 | Western Hemisphere [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 8.20% |
| William Blair International Growth Fund - Class R6 | Europe Ex-U.K. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 35.70% |
| William Blair International Growth Fund - Class R6 | Emerging Asia [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 19.60% |
| William Blair International Growth Fund - Class R6 | Asia Ex-Japan [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 4.70% |
| William Blair International Growth Fund - Class R6 | Emerging Latin America [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.40% |
| William Blair International Growth Fund - Class R6 | Emerging Europe, Mid-East, Africa [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.10% |
| William Blair Institutional International Growth Fund - Institutional Class | Taiwan Semiconductor Manufacturing Co. Ltd. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 3.90% |
| William Blair Institutional International Growth Fund - Institutional Class | Hermes International SCA [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.30% |
| William Blair Institutional International Growth Fund - Institutional Class | SAP SE [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.70% |
| William Blair Institutional International Growth Fund - Institutional Class | London Stock Exchange Group PLC [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.30% |
| William Blair Institutional International Growth Fund - Institutional Class | Tencent Holdings Ltd. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.10% |
| William Blair Institutional International Growth Fund - Institutional Class | Threei Group PLC [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.60% |
| William Blair Institutional International Growth Fund - Institutional Class | Lonza Group AG [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.20% |
| William Blair Institutional International Growth Fund - Institutional Class | DSV AS [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.20% |
| William Blair Institutional International Growth Fund - Institutional Class | Bank Central Asia Tbk. PT [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.20% |
| William Blair Institutional International Growth Fund - Institutional Class | MTU Aero Engines AG [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.20% |
| William Blair Institutional International Growth Fund - Institutional Class | Information Technology Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 18.60% |
| William Blair Institutional International Growth Fund - Institutional Class | Consumer Discretionary Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 14.80% |
| William Blair Institutional International Growth Fund - Institutional Class | Industrial Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 23.80% |
| William Blair Institutional International Growth Fund - Institutional Class | Healthcare Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 10.40% |
| William Blair Institutional International Growth Fund - Institutional Class | Financial Services Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 16.90% |
| William Blair Institutional International Growth Fund - Institutional Class | Communications Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 4.40% |
| William Blair Institutional International Growth Fund - Institutional Class | Consumer Staples Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 3.70% |
| William Blair Institutional International Growth Fund - Institutional Class | Energy Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.30% |
| William Blair Institutional International Growth Fund - Institutional Class | Materials Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2% |
| William Blair Institutional International Growth Fund - Institutional Class | Real Estate Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 0.80% |
| William Blair Institutional International Growth Fund - Institutional Class | UNITED KINGDOM | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 12.90% |
| William Blair Institutional International Growth Fund - Institutional Class | JAPAN | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 13% |
| William Blair Institutional International Growth Fund - Institutional Class | Western Hemisphere [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 8.20% |
| William Blair Institutional International Growth Fund - Institutional Class | Europe Ex-U.K. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 35.70% |
| William Blair Institutional International Growth Fund - Institutional Class | Emerging Asia [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 19.70% |
| William Blair Institutional International Growth Fund - Institutional Class | Asia Ex-Japan [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 4.70% |
| William Blair Institutional International Growth Fund - Institutional Class | Emerging Latin America [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.40% |
| William Blair Institutional International Growth Fund - Institutional Class | Emerging Europe, Mid-East, Africa [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.10% |
| William Blair International Small Cap Growth Fund Class I | Pro Medicus Ltd. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 3.30% |
| William Blair International Small Cap Growth Fund Class I | Technology One Ltd. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.60% |
| William Blair International Small Cap Growth Fund Class I | Rakuten Bank Ltd. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2% |
| William Blair International Small Cap Growth Fund Class I | Diploma PLC [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2% |
| William Blair International Small Cap Growth Fund Class I | Trustpilot Group PLC [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2% |
| William Blair International Small Cap Growth Fund Class I | North West Co., Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.80% |
| William Blair International Small Cap Growth Fund Class I | Lifco AB [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.70% |
| William Blair International Small Cap Growth Fund Class I | Beazley PLC [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.70% |
| William Blair International Small Cap Growth Fund Class I | Lotes Co. Ltd. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.60% |
| William Blair International Small Cap Growth Fund Class I | BayCurrent Consulting, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.60% |
| William Blair International Small Cap Growth Fund Class I | Information Technology Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 15.90% |
| William Blair International Small Cap Growth Fund Class I | Consumer Discretionary Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 8.50% |
| William Blair International Small Cap Growth Fund Class I | Industrial Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 28.20% |
| William Blair International Small Cap Growth Fund Class I | Healthcare Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 9.50% |
| William Blair International Small Cap Growth Fund Class I | Financial Services Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 19.50% |
| William Blair International Small Cap Growth Fund Class I | Communications Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 7.10% |
| William Blair International Small Cap Growth Fund Class I | Consumer Staples Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 7.80% |
| William Blair International Small Cap Growth Fund Class I | Energy Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 0.40% |
| William Blair International Small Cap Growth Fund Class I | Materials Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.40% |
| William Blair International Small Cap Growth Fund Class I | UNITED KINGDOM | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 11.10% |
| William Blair International Small Cap Growth Fund Class I | JAPAN | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 16.80% |
| William Blair International Small Cap Growth Fund Class I | Western Hemisphere [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 5.20% |
| William Blair International Small Cap Growth Fund Class I | Europe Ex-U.K. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 28.40% |
| William Blair International Small Cap Growth Fund Class I | Emerging Asia [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 20.40% |
| William Blair International Small Cap Growth Fund Class I | Asia Ex-Japan [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 8.90% |
| William Blair International Small Cap Growth Fund Class I | Emerging Latin America [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 3.40% |
| William Blair International Small Cap Growth Fund Class I | EMEA [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 4.10% |
| William Blair International Small Cap Growth Fund - Class N | Pro Medicus Ltd. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 3.30% |
| William Blair International Small Cap Growth Fund - Class N | Technology One Ltd. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.60% |
| William Blair International Small Cap Growth Fund - Class N | Rakuten Bank Ltd. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2% |
| William Blair International Small Cap Growth Fund - Class N | Diploma PLC [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2% |
| William Blair International Small Cap Growth Fund - Class N | Trustpilot Group PLC [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2% |
| William Blair International Small Cap Growth Fund - Class N | North West Co., Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.80% |
| William Blair International Small Cap Growth Fund - Class N | Lifco AB [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.70% |
| William Blair International Small Cap Growth Fund - Class N | Beazley PLC [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.70% |
| William Blair International Small Cap Growth Fund - Class N | Lotes Co. Ltd. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.60% |
| William Blair International Small Cap Growth Fund - Class N | BayCurrent Consulting, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.60% |
| William Blair International Small Cap Growth Fund - Class N | Information Technology Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 15.90% |
| William Blair International Small Cap Growth Fund - Class N | Consumer Discretionary Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 8.50% |
| William Blair International Small Cap Growth Fund - Class N | Industrial Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 28.20% |
| William Blair International Small Cap Growth Fund - Class N | Healthcare Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 9.50% |
| William Blair International Small Cap Growth Fund - Class N | Financial Services Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 19.50% |
| William Blair International Small Cap Growth Fund - Class N | Communications Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 7.10% |
| William Blair International Small Cap Growth Fund - Class N | Consumer Staples Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 7.80% |
| William Blair International Small Cap Growth Fund - Class N | Energy Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 0.40% |
| William Blair International Small Cap Growth Fund - Class N | Materials Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.40% |
| William Blair International Small Cap Growth Fund - Class N | UNITED KINGDOM | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 11.10% |
| William Blair International Small Cap Growth Fund - Class N | JAPAN | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 16.80% |
| William Blair International Small Cap Growth Fund - Class N | Western Hemisphere [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 5.20% |
| William Blair International Small Cap Growth Fund - Class N | Europe Ex-U.K. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 28.40% |
| William Blair International Small Cap Growth Fund - Class N | Emerging Asia [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 20.40% |
| William Blair International Small Cap Growth Fund - Class N | Asia Ex-Japan [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 8.90% |
| William Blair International Small Cap Growth Fund - Class N | Emerging Latin America [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 3.40% |
| William Blair International Small Cap Growth Fund - Class N | EMEA [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 4.10% |
| William Blair International Small Cap Growth Fund - Class R6 | Pro Medicus Ltd. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 3.30% |
| William Blair International Small Cap Growth Fund - Class R6 | Technology One Ltd. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.60% |
| William Blair International Small Cap Growth Fund - Class R6 | Rakuten Bank Ltd. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2% |
| William Blair International Small Cap Growth Fund - Class R6 | Diploma PLC [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2% |
| William Blair International Small Cap Growth Fund - Class R6 | Trustpilot Group PLC [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2% |
| William Blair International Small Cap Growth Fund - Class R6 | North West Co., Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.80% |
| William Blair International Small Cap Growth Fund - Class R6 | Lifco AB [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.70% |
| William Blair International Small Cap Growth Fund - Class R6 | Beazley PLC [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.70% |
| William Blair International Small Cap Growth Fund - Class R6 | Lotes Co. Ltd. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.60% |
| William Blair International Small Cap Growth Fund - Class R6 | BayCurrent Consulting, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.60% |
| William Blair International Small Cap Growth Fund - Class R6 | Information Technology Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 15.90% |
| William Blair International Small Cap Growth Fund - Class R6 | Consumer Discretionary Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 8.50% |
| William Blair International Small Cap Growth Fund - Class R6 | Industrial Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 28.20% |
| William Blair International Small Cap Growth Fund - Class R6 | Healthcare Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 9.50% |
| William Blair International Small Cap Growth Fund - Class R6 | Financial Services Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 19.50% |
| William Blair International Small Cap Growth Fund - Class R6 | Communications Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 7.10% |
| William Blair International Small Cap Growth Fund - Class R6 | Consumer Staples Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 7.80% |
| William Blair International Small Cap Growth Fund - Class R6 | Energy Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 0.40% |
| William Blair International Small Cap Growth Fund - Class R6 | Materials Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.40% |
| William Blair International Small Cap Growth Fund - Class R6 | UNITED KINGDOM | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 11.10% |
| William Blair International Small Cap Growth Fund - Class R6 | JAPAN | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 16.80% |
| William Blair International Small Cap Growth Fund - Class R6 | Western Hemisphere [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 5.20% |
| William Blair International Small Cap Growth Fund - Class R6 | Europe Ex-U.K. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 28.40% |
| William Blair International Small Cap Growth Fund - Class R6 | Emerging Asia [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 20.40% |
| William Blair International Small Cap Growth Fund - Class R6 | Asia Ex-Japan [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 8.90% |
| William Blair International Small Cap Growth Fund - Class R6 | Emerging Latin America [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 3.40% |
| William Blair International Small Cap Growth Fund - Class R6 | EMEA [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 4.10% |
| William Blair Emerging Markets Leaders Fund - Class I | Taiwan Semiconductor Manufacturing Co. Ltd. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 10.90% |
| William Blair Emerging Markets Leaders Fund - Class I | MediaTek, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 3% |
| William Blair Emerging Markets Leaders Fund - Class I | Tencent Holdings Ltd. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 6.50% |
| William Blair Emerging Markets Leaders Fund - Class I | Bank Central Asia Tbk. PT [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.30% |
| William Blair Emerging Markets Leaders Fund - Class I | Trip.com Group Ltd. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 4% |
| William Blair Emerging Markets Leaders Fund - Class I | Kotak Mahindra Bank Ltd. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.50% |
| William Blair Emerging Markets Leaders Fund - Class I | Meituan [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.50% |
| William Blair Emerging Markets Leaders Fund - Class I | SK Hynix, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.20% |
| William Blair Emerging Markets Leaders Fund - Class I | Contemporary Amperex Technology Co. Ltd. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.10% |
| William Blair Emerging Markets Leaders Fund - Class I | Delta Electronics, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2% |
| William Blair Emerging Markets Leaders Fund - Class I | Information Technology Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 29.70% |
| William Blair Emerging Markets Leaders Fund - Class I | Consumer Discretionary Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 16.20% |
| William Blair Emerging Markets Leaders Fund - Class I | Industrial Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 9.90% |
| William Blair Emerging Markets Leaders Fund - Class I | Healthcare Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.30% |
| William Blair Emerging Markets Leaders Fund - Class I | Financial Services Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 17.50% |
| William Blair Emerging Markets Leaders Fund - Class I | Communications Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 9.90% |
| William Blair Emerging Markets Leaders Fund - Class I | Consumer Staples Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 7.90% |
| William Blair Emerging Markets Leaders Fund - Class I | Energy Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.50% |
| William Blair Emerging Markets Leaders Fund - Class I | Materials Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2% |
| William Blair Emerging Markets Leaders Fund - Class I | Real Estate Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.90% |
| William Blair Emerging Markets Leaders Fund - Class I | Emerging Asia [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 81.20% |
| William Blair Emerging Markets Leaders Fund - Class I | Asia Ex-Japan [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.60% |
| William Blair Emerging Markets Leaders Fund - Class I | Emerging Latin America [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 10.70% |
| William Blair Emerging Markets Leaders Fund - Class I | EMEA [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 6.30% |
| William Blair Emerging Markets Leaders Fund - Class N | Taiwan Semiconductor Manufacturing Co. Ltd. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 10.90% |
| William Blair Emerging Markets Leaders Fund - Class N | MediaTek, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 3% |
| William Blair Emerging Markets Leaders Fund - Class N | Tencent Holdings Ltd. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 6.50% |
| William Blair Emerging Markets Leaders Fund - Class N | Bank Central Asia Tbk. PT [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.30% |
| William Blair Emerging Markets Leaders Fund - Class N | Trip.com Group Ltd. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 4% |
| William Blair Emerging Markets Leaders Fund - Class N | Kotak Mahindra Bank Ltd. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.50% |
| William Blair Emerging Markets Leaders Fund - Class N | Meituan [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.50% |
| William Blair Emerging Markets Leaders Fund - Class N | SK Hynix, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.20% |
| William Blair Emerging Markets Leaders Fund - Class N | Contemporary Amperex Technology Co. Ltd. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.10% |
| William Blair Emerging Markets Leaders Fund - Class N | Delta Electronics, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2% |
| William Blair Emerging Markets Leaders Fund - Class N | Information Technology Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 29.70% |
| William Blair Emerging Markets Leaders Fund - Class N | Consumer Discretionary Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 16.20% |
| William Blair Emerging Markets Leaders Fund - Class N | Industrial Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 9.90% |
| William Blair Emerging Markets Leaders Fund - Class N | Healthcare Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.30% |
| William Blair Emerging Markets Leaders Fund - Class N | Financial Services Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 17.50% |
| William Blair Emerging Markets Leaders Fund - Class N | Communications Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 9.90% |
| William Blair Emerging Markets Leaders Fund - Class N | Consumer Staples Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 7.90% |
| William Blair Emerging Markets Leaders Fund - Class N | Energy Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.50% |
| William Blair Emerging Markets Leaders Fund - Class N | Materials Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2% |
| William Blair Emerging Markets Leaders Fund - Class N | Real Estate Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.90% |
| William Blair Emerging Markets Leaders Fund - Class N | Emerging Asia [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 81.20% |
| William Blair Emerging Markets Leaders Fund - Class N | Asia Ex-Japan [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.60% |
| William Blair Emerging Markets Leaders Fund - Class N | Emerging Latin America [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 10.70% |
| William Blair Emerging Markets Leaders Fund - Class N | EMEA [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 6.30% |
| William Blair Emerging Markets Leaders Fund - Class R6 | Taiwan Semiconductor Manufacturing Co. Ltd. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 10.90% |
| William Blair Emerging Markets Leaders Fund - Class R6 | MediaTek, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 3% |
| William Blair Emerging Markets Leaders Fund - Class R6 | Tencent Holdings Ltd. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 6.50% |
| William Blair Emerging Markets Leaders Fund - Class R6 | Bank Central Asia Tbk. PT [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.30% |
| William Blair Emerging Markets Leaders Fund - Class R6 | Trip.com Group Ltd. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 4% |
| William Blair Emerging Markets Leaders Fund - Class R6 | Kotak Mahindra Bank Ltd. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.50% |
| William Blair Emerging Markets Leaders Fund - Class R6 | Meituan [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.50% |
| William Blair Emerging Markets Leaders Fund - Class R6 | SK Hynix, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.20% |
| William Blair Emerging Markets Leaders Fund - Class R6 | Contemporary Amperex Technology Co. Ltd. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.10% |
| William Blair Emerging Markets Leaders Fund - Class R6 | Delta Electronics, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2% |
| William Blair Emerging Markets Leaders Fund - Class R6 | Information Technology Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 29.70% |
| William Blair Emerging Markets Leaders Fund - Class R6 | Consumer Discretionary Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 16.20% |
| William Blair Emerging Markets Leaders Fund - Class R6 | Industrial Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 9.90% |
| William Blair Emerging Markets Leaders Fund - Class R6 | Healthcare Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.30% |
| William Blair Emerging Markets Leaders Fund - Class R6 | Financial Services Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 17.50% |
| William Blair Emerging Markets Leaders Fund - Class R6 | Communications Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 9.90% |
| William Blair Emerging Markets Leaders Fund - Class R6 | Consumer Staples Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 7.90% |
| William Blair Emerging Markets Leaders Fund - Class R6 | Energy Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.50% |
| William Blair Emerging Markets Leaders Fund - Class R6 | Materials Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2% |
| William Blair Emerging Markets Leaders Fund - Class R6 | Real Estate Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.90% |
| William Blair Emerging Markets Leaders Fund - Class R6 | Emerging Asia [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 81.20% |
| William Blair Emerging Markets Leaders Fund - Class R6 | Asia Ex-Japan [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.60% |
| William Blair Emerging Markets Leaders Fund - Class R6 | Emerging Latin America [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 10.70% |
| William Blair Emerging Markets Leaders Fund - Class R6 | EMEA [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 6.30% |
| William Blair Emerging Markets Growth Fund - Class I | Taiwan Semiconductor Manufacturing Co. Ltd. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 13.30% |
| William Blair Emerging Markets Growth Fund - Class I | Tencent Holdings Ltd. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 6.70% |
| William Blair Emerging Markets Growth Fund - Class I | Bank Central Asia Tbk. PT [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.50% |
| William Blair Emerging Markets Growth Fund - Class I | Trip.com Group Ltd. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 3.60% |
| William Blair Emerging Markets Growth Fund - Class I | Meituan [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 3.30% |
| William Blair Emerging Markets Growth Fund - Class I | SK Hynix, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.50% |
| William Blair Emerging Markets Growth Fund - Class I | MercadoLibre, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.30% |
| William Blair Emerging Markets Growth Fund - Class I | Sea Ltd. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.80% |
| William Blair Emerging Markets Growth Fund - Class I | Xiaomi Corporation [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.70% |
| William Blair Emerging Markets Growth Fund - Class I | ICICI Bank Ltd. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.50% |
| William Blair Emerging Markets Growth Fund - Class I | Information Technology Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 28.80% |
| William Blair Emerging Markets Growth Fund - Class I | Consumer Discretionary Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 20.20% |
| William Blair Emerging Markets Growth Fund - Class I | Industrial Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 10.90% |
| William Blair Emerging Markets Growth Fund - Class I | Healthcare Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 3.70% |
| William Blair Emerging Markets Growth Fund - Class I | Financial Services Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 16.20% |
| William Blair Emerging Markets Growth Fund - Class I | Communications Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 9.50% |
| William Blair Emerging Markets Growth Fund - Class I | Consumer Staples Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 5.10% |
| William Blair Emerging Markets Growth Fund - Class I | Energy Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1% |
| William Blair Emerging Markets Growth Fund - Class I | Materials Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 0.20% |
| William Blair Emerging Markets Growth Fund - Class I | Real Estate Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 3.60% |
| William Blair Emerging Markets Growth Fund - Class I | Utilities Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.50% |
| William Blair Emerging Markets Growth Fund - Class I | Emerging Asia [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 81.30% |
| William Blair Emerging Markets Growth Fund - Class I | Asia Ex-Japan [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.90% |
| William Blair Emerging Markets Growth Fund - Class I | Emerging Latin America [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 8.50% |
| William Blair Emerging Markets Growth Fund - Class I | EMEA [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 9% |
| William Blair Emerging Markets Growth Fund - Class N | Taiwan Semiconductor Manufacturing Co. Ltd. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 13.30% |
| William Blair Emerging Markets Growth Fund - Class N | Tencent Holdings Ltd. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 6.70% |
| William Blair Emerging Markets Growth Fund - Class N | Bank Central Asia Tbk. PT [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.50% |
| William Blair Emerging Markets Growth Fund - Class N | Trip.com Group Ltd. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 3.60% |
| William Blair Emerging Markets Growth Fund - Class N | Meituan [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 3.30% |
| William Blair Emerging Markets Growth Fund - Class N | SK Hynix, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.50% |
| William Blair Emerging Markets Growth Fund - Class N | MercadoLibre, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.30% |
| William Blair Emerging Markets Growth Fund - Class N | Sea Ltd. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.80% |
| William Blair Emerging Markets Growth Fund - Class N | Xiaomi Corporation [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.70% |
| William Blair Emerging Markets Growth Fund - Class N | ICICI Bank Ltd. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.50% |
| William Blair Emerging Markets Growth Fund - Class N | Information Technology Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 28.80% |
| William Blair Emerging Markets Growth Fund - Class N | Consumer Discretionary Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 20.20% |
| William Blair Emerging Markets Growth Fund - Class N | Industrial Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 10.90% |
| William Blair Emerging Markets Growth Fund - Class N | Healthcare Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 3.70% |
| William Blair Emerging Markets Growth Fund - Class N | Financial Services Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 16.20% |
| William Blair Emerging Markets Growth Fund - Class N | Communications Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 9.50% |
| William Blair Emerging Markets Growth Fund - Class N | Consumer Staples Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 5.10% |
| William Blair Emerging Markets Growth Fund - Class N | Energy Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1% |
| William Blair Emerging Markets Growth Fund - Class N | Materials Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 0.20% |
| William Blair Emerging Markets Growth Fund - Class N | Real Estate Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 3.60% |
| William Blair Emerging Markets Growth Fund - Class N | Utilities Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.50% |
| William Blair Emerging Markets Growth Fund - Class N | Emerging Asia [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 81.30% |
| William Blair Emerging Markets Growth Fund - Class N | Asia Ex-Japan [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.90% |
| William Blair Emerging Markets Growth Fund - Class N | Emerging Latin America [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 8.50% |
| William Blair Emerging Markets Growth Fund - Class N | EMEA [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 9% |
| William Blair Emerging Markets Growth Fund - Class R6 | Taiwan Semiconductor Manufacturing Co. Ltd. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 13.30% |
| William Blair Emerging Markets Growth Fund - Class R6 | Tencent Holdings Ltd. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 6.70% |
| William Blair Emerging Markets Growth Fund - Class R6 | Bank Central Asia Tbk. PT [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.50% |
| William Blair Emerging Markets Growth Fund - Class R6 | Trip.com Group Ltd. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 3.60% |
| William Blair Emerging Markets Growth Fund - Class R6 | Meituan [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 3.30% |
| William Blair Emerging Markets Growth Fund - Class R6 | SK Hynix, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.50% |
| William Blair Emerging Markets Growth Fund - Class R6 | MercadoLibre, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.30% |
| William Blair Emerging Markets Growth Fund - Class R6 | Sea Ltd. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.80% |
| William Blair Emerging Markets Growth Fund - Class R6 | Xiaomi Corporation [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.70% |
| William Blair Emerging Markets Growth Fund - Class R6 | ICICI Bank Ltd. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.50% |
| William Blair Emerging Markets Growth Fund - Class R6 | Information Technology Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 28.80% |
| William Blair Emerging Markets Growth Fund - Class R6 | Consumer Discretionary Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 20.20% |
| William Blair Emerging Markets Growth Fund - Class R6 | Industrial Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 10.90% |
| William Blair Emerging Markets Growth Fund - Class R6 | Healthcare Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 3.70% |
| William Blair Emerging Markets Growth Fund - Class R6 | Financial Services Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 16.20% |
| William Blair Emerging Markets Growth Fund - Class R6 | Communications Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 9.50% |
| William Blair Emerging Markets Growth Fund - Class R6 | Consumer Staples Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 5.10% |
| William Blair Emerging Markets Growth Fund - Class R6 | Energy Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1% |
| William Blair Emerging Markets Growth Fund - Class R6 | Materials Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 0.20% |
| William Blair Emerging Markets Growth Fund - Class R6 | Real Estate Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 3.60% |
| William Blair Emerging Markets Growth Fund - Class R6 | Utilities Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.50% |
| William Blair Emerging Markets Growth Fund - Class R6 | Emerging Asia [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 81.30% |
| William Blair Emerging Markets Growth Fund - Class R6 | Asia Ex-Japan [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.90% |
| William Blair Emerging Markets Growth Fund - Class R6 | Emerging Latin America [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 8.50% |
| William Blair Emerging Markets Growth Fund - Class R6 | EMEA [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 9% |
| William Blair Emerging Markets ex China Growth Fund - Class 1 | Taiwan Semiconductor Manufacturing Co. Ltd. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 16.50% |
| William Blair Emerging Markets ex China Growth Fund - Class 1 | MediaTek, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.90% |
| William Blair Emerging Markets ex China Growth Fund - Class 1 | Bank Central Asia Tbk. PT [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.80% |
| William Blair Emerging Markets ex China Growth Fund - Class 1 | SK Hynix, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.50% |
| William Blair Emerging Markets ex China Growth Fund - Class 1 | MercadoLibre, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.90% |
| William Blair Emerging Markets ex China Growth Fund - Class 1 | Sea Ltd. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.60% |
| William Blair Emerging Markets ex China Growth Fund - Class 1 | Emaar Properties PJSC [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.50% |
| William Blair Emerging Markets ex China Growth Fund - Class 1 | Dixon Technologies India Ltd. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.50% |
| William Blair Emerging Markets ex China Growth Fund - Class 1 | Trent Ltd. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.40% |
| William Blair Emerging Markets ex China Growth Fund - Class 1 | Indian Hotels Co. Ltd. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.40% |
| William Blair Emerging Markets ex China Growth Fund - Class 1 | Information Technology Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 33.30% |
| William Blair Emerging Markets ex China Growth Fund - Class 1 | Consumer Discretionary Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 14.40% |
| William Blair Emerging Markets ex China Growth Fund - Class 1 | Industrial Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 15.70% |
| William Blair Emerging Markets ex China Growth Fund - Class 1 | Healthcare Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.90% |
| William Blair Emerging Markets ex China Growth Fund - Class 1 | Financial Services Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 16.10% |
| William Blair Emerging Markets ex China Growth Fund - Class 1 | Communications Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.80% |
| William Blair Emerging Markets ex China Growth Fund - Class 1 | Consumer Staples Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 4.70% |
| William Blair Emerging Markets ex China Growth Fund - Class 1 | Energy Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.90% |
| William Blair Emerging Markets ex China Growth Fund - Class 1 | Materials Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 0.40% |
| William Blair Emerging Markets ex China Growth Fund - Class 1 | Real Estate Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 5.30% |
| William Blair Emerging Markets ex China Growth Fund - Class 1 | Utilities Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.80% |
| William Blair Emerging Markets ex China Growth Fund - Class 1 | Emerging Asia [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 72.40% |
| William Blair Emerging Markets ex China Growth Fund - Class 1 | Emerging Latin America [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 10.80% |
| William Blair Emerging Markets ex China Growth Fund - Class 1 | EMEA [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 16.10% |
| William Blair Emerging Markets ex China Growth Fund - Class R6 | Taiwan Semiconductor Manufacturing Co. Ltd. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 16.50% |
| William Blair Emerging Markets ex China Growth Fund - Class R6 | MediaTek, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.90% |
| William Blair Emerging Markets ex China Growth Fund - Class R6 | Bank Central Asia Tbk. PT [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.80% |
| William Blair Emerging Markets ex China Growth Fund - Class R6 | SK Hynix, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.50% |
| William Blair Emerging Markets ex China Growth Fund - Class R6 | MercadoLibre, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.90% |
| William Blair Emerging Markets ex China Growth Fund - Class R6 | Sea Ltd. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.60% |
| William Blair Emerging Markets ex China Growth Fund - Class R6 | Emaar Properties PJSC [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.50% |
| William Blair Emerging Markets ex China Growth Fund - Class R6 | Dixon Technologies India Ltd. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.50% |
| William Blair Emerging Markets ex China Growth Fund - Class R6 | Trent Ltd. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.40% |
| William Blair Emerging Markets ex China Growth Fund - Class R6 | Indian Hotels Co. Ltd. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.40% |
| William Blair Emerging Markets ex China Growth Fund - Class R6 | Information Technology Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 33.30% |
| William Blair Emerging Markets ex China Growth Fund - Class R6 | Consumer Discretionary Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 14.40% |
| William Blair Emerging Markets ex China Growth Fund - Class R6 | Industrial Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 15.70% |
| William Blair Emerging Markets ex China Growth Fund - Class R6 | Healthcare Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.90% |
| William Blair Emerging Markets ex China Growth Fund - Class R6 | Financial Services Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 16.10% |
| William Blair Emerging Markets ex China Growth Fund - Class R6 | Communications Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.80% |
| William Blair Emerging Markets ex China Growth Fund - Class R6 | Consumer Staples Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 4.70% |
| William Blair Emerging Markets ex China Growth Fund - Class R6 | Energy Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.90% |
| William Blair Emerging Markets ex China Growth Fund - Class R6 | Materials Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 0.40% |
| William Blair Emerging Markets ex China Growth Fund - Class R6 | Real Estate Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 5.30% |
| William Blair Emerging Markets ex China Growth Fund - Class R6 | Utilities Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.80% |
| William Blair Emerging Markets ex China Growth Fund - Class R6 | Emerging Asia [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 72.40% |
| William Blair Emerging Markets ex China Growth Fund - Class R6 | Emerging Latin America [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 10.80% |
| William Blair Emerging Markets ex China Growth Fund - Class R6 | EMEA [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 16.10% |
| William Blair Emerging Markets Small Cap Growth Fund - Class 1 | Dixon Technologies India Ltd. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.30% |
| William Blair Emerging Markets Small Cap Growth Fund - Class 1 | Banco Macro SA [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.50% |
| William Blair Emerging Markets Small Cap Growth Fund - Class 1 | Emaar Development PJSC [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.20% |
| William Blair Emerging Markets Small Cap Growth Fund - Class 1 | Gamuda Bhd. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.90% |
| William Blair Emerging Markets Small Cap Growth Fund - Class 1 | Eastroc Beverage Group Co. Ltd. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.80% |
| William Blair Emerging Markets Small Cap Growth Fund - Class 1 | Bank Syariah Indonesia Tbk. PT [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.70% |
| William Blair Emerging Markets Small Cap Growth Fund - Class 1 | Salik Co. PJSC [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.60% |
| William Blair Emerging Markets Small Cap Growth Fund - Class 1 | Kaynes Technology India Ltd. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.60% |
| William Blair Emerging Markets Small Cap Growth Fund - Class 1 | Central Depository Services India Ltd. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.60% |
| William Blair Emerging Markets Small Cap Growth Fund - Class 1 | BBB Foods, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.50% |
| William Blair Emerging Markets Small Cap Growth Fund - Class 1 | Information Technology Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 14.50% |
| William Blair Emerging Markets Small Cap Growth Fund - Class 1 | Consumer Discretionary Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 14.70% |
| William Blair Emerging Markets Small Cap Growth Fund - Class 1 | Industrial Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 25.60% |
| William Blair Emerging Markets Small Cap Growth Fund - Class 1 | Healthcare Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.70% |
| William Blair Emerging Markets Small Cap Growth Fund - Class 1 | Financial Services Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 19.20% |
| William Blair Emerging Markets Small Cap Growth Fund - Class 1 | Communications Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.20% |
| William Blair Emerging Markets Small Cap Growth Fund - Class 1 | Consumer Staples Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 8.70% |
| William Blair Emerging Markets Small Cap Growth Fund - Class 1 | Energy Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 3% |
| William Blair Emerging Markets Small Cap Growth Fund - Class 1 | Materials Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.60% |
| William Blair Emerging Markets Small Cap Growth Fund - Class 1 | Real Estate Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 6.30% |
| William Blair Emerging Markets Small Cap Growth Fund - Class 1 | Utilities Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.90% |
| William Blair Emerging Markets Small Cap Growth Fund - Class 1 | Emerging Asia [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 69.50% |
| William Blair Emerging Markets Small Cap Growth Fund - Class 1 | Asia Ex-Japan [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 0.50% |
| William Blair Emerging Markets Small Cap Growth Fund - Class 1 | Emerging Latin America [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 10.90% |
| William Blair Emerging Markets Small Cap Growth Fund - Class 1 | EMEA [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 19.50% |
| William Blair Emerging Markets Small Cap Growth Fund - Class N | Dixon Technologies India Ltd. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.30% |
| William Blair Emerging Markets Small Cap Growth Fund - Class N | Banco Macro SA [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.50% |
| William Blair Emerging Markets Small Cap Growth Fund - Class N | Emaar Development PJSC [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.20% |
| William Blair Emerging Markets Small Cap Growth Fund - Class N | Gamuda Bhd. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.90% |
| William Blair Emerging Markets Small Cap Growth Fund - Class N | Eastroc Beverage Group Co. Ltd. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.80% |
| William Blair Emerging Markets Small Cap Growth Fund - Class N | Bank Syariah Indonesia Tbk. PT [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.70% |
| William Blair Emerging Markets Small Cap Growth Fund - Class N | Salik Co. PJSC [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.60% |
| William Blair Emerging Markets Small Cap Growth Fund - Class N | Kaynes Technology India Ltd. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.60% |
| William Blair Emerging Markets Small Cap Growth Fund - Class N | Central Depository Services India Ltd. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.60% |
| William Blair Emerging Markets Small Cap Growth Fund - Class N | BBB Foods, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.50% |
| William Blair Emerging Markets Small Cap Growth Fund - Class N | Information Technology Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 14.50% |
| William Blair Emerging Markets Small Cap Growth Fund - Class N | Consumer Discretionary Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 14.70% |
| William Blair Emerging Markets Small Cap Growth Fund - Class N | Industrial Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 25.60% |
| William Blair Emerging Markets Small Cap Growth Fund - Class N | Healthcare Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.70% |
| William Blair Emerging Markets Small Cap Growth Fund - Class N | Financial Services Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 19.20% |
| William Blair Emerging Markets Small Cap Growth Fund - Class N | Communications Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.20% |
| William Blair Emerging Markets Small Cap Growth Fund - Class N | Consumer Staples Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 8.70% |
| William Blair Emerging Markets Small Cap Growth Fund - Class N | Energy Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 3% |
| William Blair Emerging Markets Small Cap Growth Fund - Class N | Materials Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.60% |
| William Blair Emerging Markets Small Cap Growth Fund - Class N | Real Estate Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 6.30% |
| William Blair Emerging Markets Small Cap Growth Fund - Class N | Utilities Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.90% |
| William Blair Emerging Markets Small Cap Growth Fund - Class N | Emerging Asia [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 69.50% |
| William Blair Emerging Markets Small Cap Growth Fund - Class N | Asia Ex-Japan [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 0.50% |
| William Blair Emerging Markets Small Cap Growth Fund - Class N | Emerging Latin America [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 10.90% |
| William Blair Emerging Markets Small Cap Growth Fund - Class N | EMEA [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 19.50% |
| William Blair Emerging Markets Small Cap Growth Fund - Class R6 | Dixon Technologies India Ltd. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.30% |
| William Blair Emerging Markets Small Cap Growth Fund - Class R6 | Banco Macro SA [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.50% |
| William Blair Emerging Markets Small Cap Growth Fund - Class R6 | Emaar Development PJSC [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.20% |
| William Blair Emerging Markets Small Cap Growth Fund - Class R6 | Gamuda Bhd. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.90% |
| William Blair Emerging Markets Small Cap Growth Fund - Class R6 | Eastroc Beverage Group Co. Ltd. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.80% |
| William Blair Emerging Markets Small Cap Growth Fund - Class R6 | Bank Syariah Indonesia Tbk. PT [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.70% |
| William Blair Emerging Markets Small Cap Growth Fund - Class R6 | Salik Co. PJSC [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.60% |
| William Blair Emerging Markets Small Cap Growth Fund - Class R6 | Kaynes Technology India Ltd. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.60% |
| William Blair Emerging Markets Small Cap Growth Fund - Class R6 | Central Depository Services India Ltd. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.60% |
| William Blair Emerging Markets Small Cap Growth Fund - Class R6 | BBB Foods, Inc. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.50% |
| William Blair Emerging Markets Small Cap Growth Fund - Class R6 | Information Technology Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 14.50% |
| William Blair Emerging Markets Small Cap Growth Fund - Class R6 | Consumer Discretionary Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 14.70% |
| William Blair Emerging Markets Small Cap Growth Fund - Class R6 | Industrial Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 25.60% |
| William Blair Emerging Markets Small Cap Growth Fund - Class R6 | Healthcare Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.70% |
| William Blair Emerging Markets Small Cap Growth Fund - Class R6 | Financial Services Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 19.20% |
| William Blair Emerging Markets Small Cap Growth Fund - Class R6 | Communications Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.20% |
| William Blair Emerging Markets Small Cap Growth Fund - Class R6 | Consumer Staples Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 8.70% |
| William Blair Emerging Markets Small Cap Growth Fund - Class R6 | Energy Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 3% |
| William Blair Emerging Markets Small Cap Growth Fund - Class R6 | Materials Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.60% |
| William Blair Emerging Markets Small Cap Growth Fund - Class R6 | Real Estate Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 6.30% |
| William Blair Emerging Markets Small Cap Growth Fund - Class R6 | Utilities Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.90% |
| William Blair Emerging Markets Small Cap Growth Fund - Class R6 | Emerging Asia [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 69.50% |
| William Blair Emerging Markets Small Cap Growth Fund - Class R6 | Asia Ex-Japan [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 0.50% |
| William Blair Emerging Markets Small Cap Growth Fund - Class R6 | Emerging Latin America [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 10.90% |
| William Blair Emerging Markets Small Cap Growth Fund - Class R6 | EMEA [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 19.50% |
| William Blair China Growth Fund - Class 1 | Tencent Holdings Ltd. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 13.40% |
| William Blair China Growth Fund - Class 1 | Trip.com Group Ltd. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 4.20% |
| William Blair China Growth Fund - Class 1 | Meituan [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 4.50% |
| William Blair China Growth Fund - Class 1 | Contemporary Amperex Technology Co. Ltd. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 4.50% |
| William Blair China Growth Fund - Class 1 | Xiaomi Corporation [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 4.10% |
| William Blair China Growth Fund - Class 1 | Eastroc Beverage Group Co. Ltd. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.90% |
| William Blair China Growth Fund - Class 1 | Kweichow Moutai Co. Ltd. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 5% |
| William Blair China Growth Fund - Class 1 | East Money Information Co.,Ltd. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 3.30% |
| William Blair China Growth Fund - Class 1 | BYD Company Limited [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 3.10% |
| William Blair China Growth Fund - Class 1 | Yutong Bus Co. Ltd. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.80% |
| William Blair China Growth Fund - Class 1 | Information Technology Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 13.10% |
| William Blair China Growth Fund - Class 1 | Consumer Discretionary Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 24.40% |
| William Blair China Growth Fund - Class 1 | Industrial Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 15.40% |
| William Blair China Growth Fund - Class 1 | Healthcare Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 3% |
| William Blair China Growth Fund - Class 1 | Financial Services Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 15.20% |
| William Blair China Growth Fund - Class 1 | Communications Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 15.50% |
| William Blair China Growth Fund - Class 1 | Consumer Staples Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 11.60% |
| William Blair China Growth Fund - Class 1 | Energy Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 0.80% |
| William Blair China Growth Fund - Class 1 | Utilities Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 0.60% |
| William Blair China Growth Fund - Class 1 | Emerging Asia [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 99.60% |
| William Blair China Growth Fund - Class R6 | Tencent Holdings Ltd. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 13.40% |
| William Blair China Growth Fund - Class R6 | Trip.com Group Ltd. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 4.20% |
| William Blair China Growth Fund - Class R6 | Meituan [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 4.50% |
| William Blair China Growth Fund - Class R6 | Contemporary Amperex Technology Co. Ltd. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 4.50% |
| William Blair China Growth Fund - Class R6 | Xiaomi Corporation [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 4.10% |
| William Blair China Growth Fund - Class R6 | Eastroc Beverage Group Co. Ltd. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.90% |
| William Blair China Growth Fund - Class R6 | Kweichow Moutai Co. Ltd. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 5% |
| William Blair China Growth Fund - Class R6 | East Money Information Co.,Ltd. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 3.30% |
| William Blair China Growth Fund - Class R6 | BYD Company Limited [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 3.10% |
| William Blair China Growth Fund - Class R6 | Yutong Bus Co. Ltd. [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.80% |
| William Blair China Growth Fund - Class R6 | Information Technology Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 13.10% |
| William Blair China Growth Fund - Class R6 | Consumer Discretionary Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 24.40% |
| William Blair China Growth Fund - Class R6 | Industrial Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 15.40% |
| William Blair China Growth Fund - Class R6 | Healthcare Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 3% |
| William Blair China Growth Fund - Class R6 | Financial Services Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 15.20% |
| William Blair China Growth Fund - Class R6 | Communications Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 15.50% |
| William Blair China Growth Fund - Class R6 | Consumer Staples Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 11.60% |
| William Blair China Growth Fund - Class R6 | Energy Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 0.80% |
| William Blair China Growth Fund - Class R6 | Utilities Sector [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 0.60% |
| William Blair China Growth Fund - Class R6 | Emerging Asia [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 99.60% |
| William Blair Emerging Markets Debt Fund Class I | Argentine Government International Bond [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 3.10% |
| William Blair Emerging Markets Debt Fund Class I | Egypt Government International Bond [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.90% |
| William Blair Emerging Markets Debt Fund Class I | Petroleos Mexicanos [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.80% |
| William Blair Emerging Markets Debt Fund Class I | Qatar Government International Bond [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.40% |
| William Blair Emerging Markets Debt Fund Class I | Paraguay Government International Bond [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.10% |
| William Blair Emerging Markets Debt Fund Class I | Romania Government International Bond [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.10% |
| William Blair Emerging Markets Debt Fund Class I | Dominican Republic International Bond [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.10% |
| William Blair Emerging Markets Debt Fund Class I | Ukraine Government International Bond [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2% |
| William Blair Emerging Markets Debt Fund Class I | South Africa Government International Bond [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.90% |
| William Blair Emerging Markets Debt Fund Class I | Oman Government International Bond [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.80% |
| William Blair Emerging Markets Debt Fund Class I | AAA [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 0.20% |
| William Blair Emerging Markets Debt Fund Class I | AA [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 5.40% |
| William Blair Emerging Markets Debt Fund Class I | A [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 4.50% |
| William Blair Emerging Markets Debt Fund Class I | BBB [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 17.60% |
| William Blair Emerging Markets Debt Fund Class I | BB [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 24.30% |
| William Blair Emerging Markets Debt Fund Class I | B [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 17.20% |
| William Blair Emerging Markets Debt Fund Class I | CCC [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 13% |
| William Blair Emerging Markets Debt Fund Class I | CC [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 5.50% |
| William Blair Emerging Markets Debt Fund Class I | D [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 3.70% |
| William Blair Emerging Markets Debt Fund Class I | Not Rated [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.80% |
| William Blair Emerging Markets Debt Fund Class I | Emerging Asia [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 13.90% |
| William Blair Emerging Markets Debt Fund Class I | Emerging Latin America [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 37.80% |
| William Blair Emerging Markets Debt Fund Class I | EMEA [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 40.70% |
| William Blair Emerging Markets Debt Fund Class R6 | Argentine Government International Bond [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 3.10% |
| William Blair Emerging Markets Debt Fund Class R6 | Egypt Government International Bond [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.90% |
| William Blair Emerging Markets Debt Fund Class R6 | Petroleos Mexicanos [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.80% |
| William Blair Emerging Markets Debt Fund Class R6 | Qatar Government International Bond [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.40% |
| William Blair Emerging Markets Debt Fund Class R6 | Paraguay Government International Bond [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.10% |
| William Blair Emerging Markets Debt Fund Class R6 | Romania Government International Bond [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.10% |
| William Blair Emerging Markets Debt Fund Class R6 | Dominican Republic International Bond [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.10% |
| William Blair Emerging Markets Debt Fund Class R6 | Ukraine Government International Bond [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2% |
| William Blair Emerging Markets Debt Fund Class R6 | South Africa Government International Bond [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.90% |
| William Blair Emerging Markets Debt Fund Class R6 | Oman Government International Bond [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.80% |
| William Blair Emerging Markets Debt Fund Class R6 | AAA [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 0.20% |
| William Blair Emerging Markets Debt Fund Class R6 | AA [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 5.40% |
| William Blair Emerging Markets Debt Fund Class R6 | A [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 4.50% |
| William Blair Emerging Markets Debt Fund Class R6 | BBB [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 17.60% |
| William Blair Emerging Markets Debt Fund Class R6 | BB [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 24.30% |
| William Blair Emerging Markets Debt Fund Class R6 | B [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 17.20% |
| William Blair Emerging Markets Debt Fund Class R6 | CCC [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 13% |
| William Blair Emerging Markets Debt Fund Class R6 | CC [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 5.50% |
| William Blair Emerging Markets Debt Fund Class R6 | D [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 3.70% |
| William Blair Emerging Markets Debt Fund Class R6 | Not Rated [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.80% |
| William Blair Emerging Markets Debt Fund Class R6 | Emerging Asia [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 13.90% |
| William Blair Emerging Markets Debt Fund Class R6 | Emerging Latin America [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 37.80% |
| William Blair Emerging Markets Debt Fund Class R6 | EMEA [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 40.70% |
