Emerging Markets Optical & Photonics ETF Equity Market Risk [Member] Optical & Photonics Companies Risk [Member] Sector Risks [Member] Information Technology Sector Risk [Member] Economic and Market Risk [Member] ETF Risks [Member] Authorized Participants, Market Makers, and Liquidity Providers Concentration Risk [Member] Cash Redemption Risk [Member] Costs of Buying or Selling Shares [Member] Shares May Trade at Prices Other Than NAV [Member] Trading [Member] Foreign Securities Risk [Member] Emerging Markets Risk [Member] Frontier Markets Risk [Member] Depositary Receipt Risks [Member] Currency Exchange Rate Risk [Member] China Investing Risks [Member] China A-Shares Investment Risk [Member] China Variable Interest Equity Investment Risk [Member] Derivatives Risk [Member] Swap Agreements [Member] Forward Contracts [Member] Concentration Risk [Member] Counterparty Risk [Member] Geographic Investment Risk [Member] Market Capitalization Risk [Member] Large-Capitalization Investing [Member] Mid-Capitalization Investing [Member] Small-Capitalization Investing [Member] Management Risk [Member] New Fund Risk [Member] Newer Sub-Adviser Risk [Member] Operational Risk [Member] U.S. Government and U.S. Agency Obligations Risk [Member] Emerging Markets Optical & Photonics ETF [Member] Emerging Markets Optical & Photonics ETF Emerging Markets Optical & Photonics ETF [Member] U.S. Government and U.S. Agency Obligations Risk. Operational Risk. Newer Sub-Adviser Risk. New Fund Risk. Management Risk. Small-Capitalization Investing. Mid-Capitalization Investing. Large-Capitalization Investing. Market Capitalization Risk. Geographic Investment Risk. Counterparty Risk. Concentration Risk. Swap Agreements. Derivatives Risk. China Variable Interest Equity Investment Risk. China A-Shares Investment Risk. China Investing Risks. Currency Exchange Rate Risk. Depositary Receipt Risks. Frontier Markets Risk. Emerging Markets Risk. Foreign Securities Risk. Trading. Shares May Trade at Prices Other Than NAV. Costs of Buying or Selling Shares. Cash Redemption Risk. Authorized Participants, Market Makers, and Liquidity Providers Concentration Risk. ETF Risks. Economic and Market Risk. Information Technology Sector Risk. Sector Risks. Optical & Photonics Companies Risk. Equity Market Risk.