Pricing data is sourced from Exchange Data International back to 2007, and from Quote Media prior to 2007.
Our pricing data covers all US securities trading on the major exchanges. It does not include OTC securities. It does include ETFs and ADRs/Foreign Issuers.
Our US pricing data is a composite price, so using the exchange prices endpoint, use the identifier ^USCOMP to bring in the composite pricing data for all US securities (excluding the OTC).
You can pull the latest end of day and historical end of day close price using the prices endpoint, historical data endpoint, or data point endpoint for these indices: http://docs.intrinio.com/master/stock-indices.