anticipate the correlation of performance of the underlying securities. Specifically, these bond insurers and rating agencies relied on historical data that did not prove predictive of residential mortgage loan performance following the 2008 crisis, which witnessed the first-ever nationwide decline in housing prices. Notably, AGM and AGC did not insure such CDOs, which has allowed Assured Guaranty to continue writing business throughout the financial crisis and ensuing recession and recovery. Impacts on individual companies; regulatory response The financial crisis precipitated many changes in the bond insurance industry, including rating agency downgrades, several companies ceasing to write new business, dramatic share value reductions, and consolidation among the insurers. The industry's primary regulators in New York also took action, as did their counterparts in Wisconsin. On November 7, 2007, ACA, the only single-A rated insurer, reported a $1 billion loss, wiping out equity and resulting in negative net worth. On November 19, ACA noted in a 10-Q that if downgraded below single-A-minus, it would have to post collateral to comply with standard insurance agreements, and that—based on current fair values—the firm would be unable to do so. On December 13, 2007, ACA's stock was delisted from the New York Stock Exchange due to low market price and negative net worth, although ACA retained its single-A rating. On December 19, 2007, the company was downgraded to triple-C by Standard & Poor's. Downgrades of major triple-A monolines began in December 2007, resulting in downgrades of thousands of municipal bonds and structured financings insured by the companies. In 2007 Warren Buffett's Berkshire Hathaway Assurance entered the market. Also during this time, credit default swap markets quoted rates for monoline default protection that would have been typical for below-investment-grade credits. Structured credit issuance ceased, and many municipal bond issuers went to market without bond insurance. By January 2008, many insured municipal and institutional bonds were trading at prices as if they were uninsured, effectively discounting the financial guaranty insurance completely. The slow reaction of the rating agencies in acknowledging this situation echoed their slow downgrading of subprime mortgage debt a year earlier. In 2008, the New York State Insurance Department (NYID) issued "Circular Letter No. 19", which described "best practices" for financial guaranty insurers, particularly relating to categories of securities that had damaged the industry in the financial crisis. In 2009, Assured Guaranty acquired FSA and subsequently renamed it Assured Guaranty Municipal ("AGM"), combining under the