Net Assets Investment Company, Portfolio Turnover C000023524 [Member] William Blair Growth Fund - CLASS I C000023523 [Member] William Blair Growth Fund - Class N C000211716 [Member] William Blair Growth Fund - Class R6 C000023539 [Member] William Blair Large Cap Growth Fund - Class I C000229252 [Member] China Growth Fund - Class I C000229253 [Member] China Growth Fund - Class R6 C000221009 [Member] Emerging Markets Debt Fund - Class I C000221008 [Member] Emerging Markets Debt Fund - Class R6 C000023538 [Member] William Blair Large Cap Growth Fund - Class N C000211719 [Member] William Blair Large Cap Growth Fund - CLASS R6 C000234386 [Member] William Blair Mid Cap Value Fund - Class I C000234387 [Member] William Blair Mid Cap Value Fund - Class R6 C000214491 [Member] William Blair Small-Mid Cap Core Fund - Class I C000214492 [Member] William Blair Small-Mid Cap Core Fund - Class R6 C000023543 [Member] William Blair Small-Mid Cap Growth Fund - Class I C000023542 [Member] William Blair Small-Mid Cap Growth Fund - Class N C000211722 [Member] William Blair International Growth Fund - Class R6 C000023546 [Member] William Blair Institutional International Growth Fund - Institutional Class C000023526 [Member] William Blair International Small Cap Growth Fund - Class I C000023525 [Member] William Blair International Small Cap Growth Fund - Class N C000023527 [Member] William Blair International Small Cap Growth Fund - Class R6 C000059867 [Member] William Blair Emerging Markets Leaders Fund - Class I C000088850 [Member] William Blair Emerging Markets Leaders Fund - Class N C000059868 [Member] William Blair Emerging Markets Leaders Fund - Class R6 C000023529 [Member] William Blair Emerging Markets Growth Fund - Class I C000023528 [Member] William Blair Emerging Markets Growth Fund - Class N C000023530 [Member] William Blair Emerging Markets Growth Fund R6 C000234384 [Member] Emerging Markets ex China Growth Fund-Class I C000234385 [Member] Emerging Markets ex China Growth Fund-Class R6 C000105753 [Member] Emerging Markets Small Cap Growth Fund - Class I C000105754 [Member] Emerging Markets Small Cap Growth Fund - Class N C000105755 [Member] Emerging Markets Small Cap Growth Fund - Class R6 C000211721 [Member] William Blair Small-Mid Cap Growth Fund - Class R6 C000243363 [Member] William Blair Small-Mid Cap Value Fund - Class I C000243364 [Member] William Blair Small-Mid Cap Value Fund - Class R6 C000023541 [Member] William Blair Small Cap Growth Fund - Class I C000023540 [Member] William Blair Small Cap Growth Fund - Class N C000211720 [Member] William Blair Small Cap Growth Fund - Class R6 C000023532 [Member] William Blair Small Cap Value Fund - Class I C000023531 [Member] William Blair Small Cap Value Fund - Class N C000211717 [Member] William Blair Small Cap Value Fund - Class R6 C000053608 [Member] William Blair Global Leaders Fund - Class I C000053607 [Member] William Blair Global Leaders Fund - CLASS N C000053609 [Member] William Blair Global Leaders Fund - CLASS R6 C000116469 [Member] William Blair International Leaders Fund - CLASS I C000116468 [Member] William Blair International Leaders Fund - CLASS N C000116470 [Member] William Blair International Leaders Fund - CLASS R6 C000023545 [Member] William Blair International Growth Fund - CLASS I C000023544 [Member] William Blair International Growth Fund - Class N MSCI All Country World IMI (net) [Member] MSCI All Country World (net) [Member] Russell 2000® Value Index [Member] Russell 3000 Index [Member] Russell 3000® Index [Member] Russell 2000 Growth Index [Member] Russell 2000® Growth Index [Member] Russell 2500 Value Index [Member] Russell 2500® Value Index [Member] Russell 2500 Growth Index [Member] Russell 2500™ Growth Index [Member] MSCI Emerging Markets Small Cap Index (net) [Member] MSCI Emerging Markets ex-China IMI (net) [Member] MSCI Emerging Markets IMI (net) [Member] MSCI Emerging Markets Index (net) [Member] MSCI All Country World ex-U.S. Small Cap Index (net) [Member] MSCI All Country World ex-U.S. IMI (net) [Member] Russell 2500 Index [Member] Russell 2500™ Index [Member] Russell Midcap Value Index [Member] Russell Midcap® Value Index [Member] Russell 1000 Growth Index [Member] Russell 1000® Growth Index [Member] Russell 1000 Index [Member] Russell 1000® Index [Member] JPMorgan Emerging Markets Bond Index (EMBI) Global Diversified Index [Member] Bloomberg Global Aggregate Index [Member] MSCI China All Shares Index (net) [Member] Russell 3000 Growth Index [Member] Russell 3000® Growth Index [Member] StandardPoors500IndexMember S&P 500® Index [Member] UNITED KINGDOM JAPAN Asia Ex-Japan [Member] Emerging Asia [Member] Emerging Europe, Mid-East, Africa [Member] Emerging Latin America [Member] Europe Ex-U.K. [Member] Western Hemisphere [Member] Energy Sector [Member] Financial Services Sector [Member] Healthcare Sector [Member] Real Estate Sector [Member] Investment Type [Axis] ASML Holding N.V. [Member] AstraZeneca PLC [Member] Roche Holding AG [Member] Safran [Member] Samsung Electronics Co., Ltd. [Member] SK hynix Inc. [Member] Sumitomo Mitsui Financial Group, Inc. [Member] Taiwan Semiconductor Manufacturing Company Limited [Member] Tokio Marine Holdings, Inc. [Member] UniCredit S.p.A. [Member] All Countries [Domain] Banco Bilbao Vizcaya Argentaria S.A. [Member] Fujikura Ltd. [Member] Hon Hai Precision Industry Co., Ltd. [Member] Prysmian S.p.A. [Member] Alphabet Inc. [Member] Amazon.com, Inc. [Member] Apple Inc. [Member] Applied Materials, Inc. [Member] Eli Lilly and Company [Member] Ibiden Co., Ltd. [Member] Microsoft Corporation [Member] NVIDIA Corporation [Member] Atlantic Union Bankshares Corporation [Member] Diodes Incorporated [Member] Eastern Bankshares, Inc. [Member] Hancock Whitney Corporation [Member] Hub Group, Inc. 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[Member] Lattice Semiconductor Corporation [Member] nVent Electric PLC [Member] Onto Innovation Inc. [Member] Sterling Infrastructure, Inc. [Member] Talen Energy Corporation [Member] Acter Group Corporation Limited [Member] Acutaas Chemicals Limited [Member] GE Vernova T And D India Limited [Member] GE Vernova T&D India Limited [Member] Grand Process Technology Corporation [Member] Kiatnakin Phatra Bank Public Company Limited [Member] Kinik Company [Member] L And K Engineering Co., Ltd. [Member] L&K Engineering Co., Ltd. [Member] Marketech International Corp. [Member] Optima bank SA [Member] PriceSmart, Inc. [Member] Communication Services [Member] Accton Technology Corporation [Member] Delta Electronics, Inc. [Member] Lotes Co., Ltd. [Member] MediaTek Inc. [Member] Samsung Electro-Mechanics Co., Ltd. [Member] SK Square Co., Ltd. [Member] Unimicron Technology Corp. [Member] Capitec Bank Holdings Limited [Member] Contemporary Amperex Technology Co.,Ltd. [Member] Samsung Life Insurance Co., Ltd. [Member] Tencent Holdings Limited [Member] Ase Technology Holding Co., Ltd. [Member] Bajaj Finance Limited [Member] Grupo Financiero Banorte, S.A.B. de C.V. [Member] HDFC Bank Limited [Member] Asahi Intecc Co., Ltd. [Member] Bufab AB [Member] Dassault Aviation [Member] Definity Financial Corporation [Member] flatexDEGIRO SE [Member] Glanbia Public Limited Company [Member] Kokusai Electric Corporation [Member] Renishaw plc [Member] SOL S.P.A. [Member] Zegona Communications plc [Member] Akamai Technologies, Inc. [Member] BorgWarner Inc. [Member] Everest Group, Ltd. [Member] IDACORP, Inc. [Member] Knight-Swift Transportation Holdings Inc. [Member] Landbridge Company LLC [Member] Entergy Corporation [Member] Globus Medical, Inc. [Member]