March 19 (Reuters) - NEW YORK (Standard & Poor's) March 19, 2012--Standard & Poor's Ratings Services said today that it has assigned its 'A-' senior unsecured debt rating to American International Group Inc.'s (AIG; A-/Stable/A-2) new issue of $2.0 billion of senior unsecured notes. We expect the senior notes to be issued in two tranches--$750 million of notes with a three-year maturity and $1.25 billion of notes with a five-year maturity. AIG is issuing this debt to pay down the last