Keith Henderson it sounds like you are using the embed option in ArcGIS Online is that correct? If so you can try appending a url parameter to the end of your src string that sets the panel open by default.
Here's an example showing this - note that the updated bit is the portion show in aqua.
<style>.embed-container {position: relative; padding-bottom: 80%; height: 0; max-width: 100%;} .embed-container iframe, .embed-container object, .embed-container iframe{position: absolute; top: 0; left: 0; width: 100%; height: 100%;} small{position: absolute; z-index: 40; bottom: 0; margin-bottom: -15px;}</style><div class="embed-container"><iframe width="500" height="400" frameborder="0" scrolling="no" marginheight="0" marginwidth="0" title="TestTrailMap" src="//jsapi.maps.arcgis.com/apps/Embed/index.html?webmap=0f15c61bf4e345198e906933b9b9cdd3&extent=-105.4144,39.934,-105.067,40.1175&zoom=true&scale=true&details=true&legend=true&active_panel=details&disable_scroll=true&theme=light&show_panel=true&popup_sidepanel=true;"></iframe></div>
There is one small bug with this issue in that if your initial display size is small (500x400) the panel will not open by default. We will get this fixed for the next release.