Charlie Mires of Allstate Insurance

With about 45% of Allstate Insurance Co.'s $18 billion municipal portfolio invested in insured bonds, portfolio manager Charlie Mires says he does a thorough analysis of each insurance company's trading value. But while he recognizes the differences, the market as a whole treats the four triple-A names identically.

"We think there are differences in the insurers, and we try to use that and reflect that in what we buy in the portfolio," said Mires, who declined to give the firm's ranking of insurance companies.

Allstate's analysts scrutinize the insurer's capital position, its underlying book of business, and its diversification of credits and sectors. An overweighting in volatile areas like health care or electrical utilities means "a little bit more downside risk," Mires said about wanting to take on additional credits for his $8 billion insured holdings.

When it comes to trading, however, there is almost no spread between the different names, said the Northbrook, Ill.-based portfolio manager. Even MBIA's exposure to defaulted bonds from AHERF did little to sway the trading value of other bonds the company backs.

"Clearly the AHERF bonds that were insured traded differently, but I wouldn't say I saw any impact across all MBIA-insured bonds," Mires said. "If there was something of that type, it was too small to see."