American International Group Inc., the bailed-out U.S. insurer, failed to report $18.7 billion of policyholder guarantees at two property-casualty subsidiaries in 2008, a Pennsylvania regulator said. National Union Fire Insurance Co. of Pittsburgh and American Home Assurance Co., which issued the guarantees to bolster other AIG units, had contingent liabilities tied to the promises of $157 billion on Dec. 31, 2008, compared with the $138.3 billion disclosed at the time, Robert Pratter, the state’s acting insurance commissioner, said today in a report.