William Blair Global Leaders Fund S000019315 [Member] MSCI All Country World Index (net) (reflects no deductions for fees, expenses or taxes) MSCI All Country World Index net [Member] MSCI All Country World Investable Market Index (net) (reflects no deduction for fees, expenses or taxes) MSCI All Country World Investable Market Index net [Member] Class N C000053607 [Member] Class I C000053608 [Member] Class R6 C000053609 [Member] Market Risks [Member] Foreign Investment Risk [Member] Currency Risk [Member] Emerging Markets Risk [Member] Geographic Risk [Member] Smaller Company Risk [Member] Share Ownership Concentration Risk [Member] Style Risk [Member] Focus Risk [Member] Operational and Technology Risk [Member] Equity Funds General Risk [Member]