Zurich Insurance Group said in a statement it "currently estimates aggregate claims in the third quarter of 2017 related to hurricanes Harvey, Irma and Maria ... to be approximately $700 million net of reinsurance and before tax."
The final loss assessment would "take time to complete due to the nature of the events," the statement said.
After tax, the losses were expected to be around $620 million.
Hurricane Harvey battered Texas and parts of Louisiana in late August, causing severe damage to property and paralyzing the country's fourth-largest city, Houston, with major flooding.
In September, Hurricane Irma struck the Florida Keys archipelago and Hurricane Maria slammed into Puerto Rico.
The losses were "not expected to have a material impact on the group's overall financial strength and capital flexibility," Zurich Insurance said.
"While significant, these events have demonstrated the effectiveness of our underwriting and the improvements made in our reinsurance programmes over the past year, which have ensured that the overall losses remain well within our overall risk tolerance," said chief executive Mario Greco.