Nationstar Mortgage Holdings Inc., a residential mortgage loan servicer and one of the top five non-bank servicers in the United States, filed on Monday with the SEC to raise up to $400 million in an initial public offering. The Lewisville, TX-based company booked $261 million in sales for the 12 months ended December 31, 2010, plans to list on the NYSE. Neither underwriter information nor pricing terms were disclosed.