What: Shares of MBIA (NYSE:MBI) are trading higher today. At one point, shares were up 20.5%, but have since moved down to gain 11.6% as of 12:15 p.m. ET.

So what: MBIA is on the move as Puerto Rico's electric utility reached a tentative agreement with bondholders to restructure $8.2 billion of debt.

MBIA noted in its most recent quarterly report that it had insured $1.35 billion of debt at par value issued by the Puerto Rico Electric Power Authority (PREPA), its largest single exposure in Puerto Rico.

Bloomberg previously reported that insurers will put up $450 million in the event of default, citing two unnamed sources. In all, PREPA has $8.2 billion of debt outstanding.

Now what: It's not exactly smooth sailing for MBIA. The bond insurer has insured nearly $4.3 billion of Puerto Rican bond issuance at par value. But an agreement to restructure one of its largest exposures will help the insurer better quantify how much capital is at risk in Puerto Rico's municipal debt debacle.