Illinois is on the hook for $1 billion in late-payment fees for the backlog of bills it’s piled up since 2015, a report Monday showed.

Comptroller Susana Mendoza released the first monthly debt-transparency review, which showed the state ended 2017 with $9.2 billion in overdue bills, down from a high of $16.7 billion before borrowing to pay down a portion of it.

With some of that debt carrying interest of up to 12 percent annually, there’s a $1.03 billion surcharge for the financial deadlock that began in July 2015 when Republican Gov. Bruce Rauner and Democrats who control the Legislature couldn’t agree on a budget for two years but kept spending anyway.

Mendoza’s office reported that it’s paid $143 million of the interest and is scheduled to pay $116 million more soon.

Of the backlog — which stood at $8.8 billion Monday — $2.5 billion had not yet been sent to the comptroller for processing by the agencies that incurred them. Agencies weren’t required to report the amounts they were still holding before the law that took effect in November, when lawmakers voted to override Rauner’s veto of the monthly update .

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But knowing what is coming from the agencies is part of why Mendoza wanted the law.

“This report will be an effective cash-management tool for my office and provides a much greater level of transparency for taxpayers and policymakers,” Mendoza said in a statement.

The report also stated that by the June 30 end of the fiscal year, Illinois will face $2.3 billion in bills that haven’t even received General Assembly approval.

This spending is likely spread across contracts that the government signed for goods or services without knowing whether money would be approved, or consists of costs for existing programs that ran higher than anticipated.

Those bills will remain at the agencies until they get approval, with many accruing late-payment interest.

“Gov. Rauner needs to immediately tell us what his detailed plan is for addressing the over $2 billion of unpaid bills that have not been appropriated,” said GOP Rep. David McSweeney of Barrington Hills, a co-sponsor of the debt-transparency law.

A spokeswoman for Rauner did not respond to a request for comment.

Rauner previously said he opposed the monthly reporting plan because he didn’t want the Democratic comptroller “micromanaging” the administration’s finances. Then he said the state lacked the technology to allow monthly reporting.

Eighty of 84 agencies complied on time, Mendoza’s office said. The biggest of the four who failed is the Department of Children and Family Services. A spokesman for DCFS did not respond to a question as to why.

Contact political writer John O’Connor at @APOConnor.