Ambac posts large 4th-quarter loss

NEW YORK -- Ambac Financial Group Inc. booked a massive loss Tuesday as mortgage-related troubles spread, but the bond insurer assured investors it remains a viable company even as its business slows. "We just got too complex," Michael Callen, interim chief executive, said of his company's foray into bonds backed by risky mortgages. The fallout from that bet led to a $3.26 billion fourth-quarter loss.