Wall Street Journal: Default Fears Unnerve Markets

The turmoil on Wall Street is beginning to rock a foundation of the financial system: the ability of institutions to make good on their many trades with one another.
Today, a struggling bond insurer, ACA Financial Guaranty Corp., will ask its trading partners for more time as it scrambles to unwind more than $60 billion of insurance contracts it sold to financial firms but can't fully pay off, according to people familiar with the matter. The contracts were intended to protect Wall Street firms from losses on mortgage securities and other debt they own.

The problem is that the insurer itself is teetering -- with repercussions across the financial world. Some of its trading partners, called counterparties, already are writing off billions of dollars because of its inability to pay.