United Guaranty, the market leader in private mortgage insurance – and a key partner of the GSEs – made it official this week, filing for an initial public offering of common stock in a transaction valued at $100 million. UG’s parent company American International Group will retain ownership of 80.1 percent of the firm, with the public getting the balance. As the company notes in its IPO documents, Fannie Mae and Freddie Mac must approve the spin-off. But when that will happen remains unclear. No specifics are addressed in the S-1 filing with the Securities and Exchange Commission and a spokesman for UG declined to comment on the matter.