Iowa’s largest health insurance company is announcing plans to significantly raise rates in April by as much as 25%. Iowa Senator Chuck Grassley says he’s sending a letter today to the C.E.O. of Wellmark Blue Cross Blue Shield, asking for justification of the rate increases that are being planned.

“I’m getting questions from Iowans about this and I need to know the basis for Wellmark making these decisions,” Grassley says. “What factors were considered? Did they have individual actuarial analysis? We need data on their reserves.”

Some 80,000 Iowans buy their own health insurance through Wellmark and their rates are expected to rise between 10% and 25 %. A company spokesman has said the rate hikes are due to increasing health care costs, but Grassley says he wants more detail from Wellmark, though people in several other states are facing similar premium increases.

“I think we’re entitled to documentation of these insurers’ efforts and how it relates to their seeking to be fully nonpartisan status, but my purpose is to study what’s going on,” Grassley says. “I want an informed policy debate on these issues.”

Wellmark raised insurance rates for individual policyholders last year by more than 9%. Overall, Wellmark insures 1.8 million Iowans, with most of them buying their health care coverage through their employers. Grassley, a Republican, says he wants to see the numbers.

“I’m asking because Iowa consumers deserve to know and as the health care debate in Washington continues, insurance rates are a major issue,” Grassley says. “The legislation passed last year didn’t drive costs down. In fact, premiums would’ve gone up.”

Wellmark is constructing a new headquarters in downtown Des Moines that’s expected to cost $194-million, but company officials say the structure has no impact on its rates. The move will gather workers from five existing buildings into one central location.