The XBRL taxonomy for the Federal Energy Regulatory Commission (FERC) can be found here. There are about 650 companies that file reports to FERC using XBRL. There are about 11 different types of forms that could be filed.
You can look at the FERC taxonomies here.
FERC is not using extensions. Taxonomies essentially follow the structure of each form. Currently they have about 10 forms. Each form has schedules. Every schedule has a hypercube (they use the term Table). Each reported fact is associated to at least one hypercube.