Nearly 36 hours after ploughing ashore from the Gulf of Mexico at Keaton Beach in Florida's Big Bend region, packing Category 3 winds of nearly 200km/h, Idalia weakened on Thursday from a tropical storm to a post-tropical cyclone and drifted out into the Atlantic.
At the height of its fury on Wednesday, Idalia ravaged a wide swath of low-lying and largely rural Gulf Coast landscape and forced emergency teams, some in boats, to rescue dozens of residents who became trapped by floodwaters.
The storm brought fierce winds and drove surging seawater kilometres inland, strewing the area with fallen trees, power lines and debris. Many buildings were in shambles, and power outages were widespread.
The storm ranked as the most powerful hurricane in more than a century to strike the Big Bend region, a sparsely populated area laced with marshland, rivers and springs where the state's northern Gulf Coast panhandle curves into the western side of the Florida Peninsula.
The damage and loss of life were less than many had feared, with authorities confirming three traffic-related fatalities linked to the storm in Florida and another in southeastern Georgia.
Idalia's storm surge, considered the greatest hazard posed by major hurricanes, appeared to have caused no deaths.
Even as Idalia headed out to the Atlantic, the back end of the storm system was producing downpours that were forecast to dump as much as 25cm of rain in some spots along the coastline of North and South Carolina, the National Weather Service said.
Forecasters had warned of possible life-threatening flash floods in the Carolinas. But local media reports at day's end said both states had mostly been spared.
Much of Florida's Big Bend coast was much less fortunate.
Horseshoe Beach, a community about 30 miles south of landfall, was among those that bore the brunt of Idalia's impact. Video footage showed scattered remnants of trailer homes sheared from bare concrete foundations. Other trailer homes had toppled and slid into lagoons, and boat docks were reduced to piles of splintered lumber.
Local, state and federal authorities said they would assess the full extent of damage in the days ahead. Insured property losses in Florida were projected to run to $US9.36 billion ($A14.43 billion), according to investment bank UBS.
President Joe Biden approved a major disaster declaration for several hard-hit Florida counties, and he plans to visit some of the storm-battered areas on Saturday.
Despite heavy damage to homes in many coastal communities, Idalia proved far less destructive, or lethal, than Hurricane Ian, a Category 5 storm that struck Florida last September, killing 150 people and causing $US112 billion ($A173 billion) in property losses.
Across the Southeast, electricity outages from fallen trees, utility poles and power lines were widespread. In all, more than 175,000 homes and businesses were without power in Florida, Georgia and the Carolinas on Thursday.
Florida officials said crews would restore most of the state's electricity within 48 hours.
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