| Company | Ranking | Outlook | Date |
|---|---|---|---|
| DBRS | AAA | Secure | Apr 17 2012 |
| Fitch | AAA | Secure | Mar 08 2004 |
| S&P | AAA | Secure | Feb 16 2004 |
| Moody’s | Aaa | Secure | Apr 04 2002 |
| Fitch | AA+ | Secure | Mar 04 2002 |
| Fitch | AA | Constructive | Jul 18 2001 |
| Fitch | AA | Secure | Sep 21 2000 |
| S&P | AA+ | Constructive | Jul 24 2000 |
| Moody’s | Aa1 | Secure | Aug 23 1999 |
| Moody’s | Aa2 | Constructive | Feb 10 1999 |
| Fitch | AA | N/A | Jan 27 1999 |
| Moody’s | Aa2 | Secure | Jun 04 1998 |
| Moody’s | Aa3 | Constructive Watch | Might 15 1998 |
| Moody’s | Aa3 | Constructive | Jun 26 1997 |
| S&P | AA+ | Secure | Apr 18 1997 |
| S&P | AA+ | Unfavorable | Jan 12 1995 |
| Moody’s | Aa3 | Secure | Jan 05 1995 |
| Fitch | AA- | N/A | Dec 22 1994 |
| Moody’s | Aa2 | Unfavorable Watch | Oct 08 1994 |
| Fitch | AA | N/A | Aug 10 1994 |
| S&P | AA+ | Secure | Mar 23 1993 |
| S&P | AAA | Secure | Mar 22 1993 |
| Moody’s | Aa2 | Secure | Feb 01 1993 |
| Moody’s | Aa1 | Secure | Jan 17 1991 |
| S&P | AAA | Unfavorable | Oct 31 1990 |
| S&P | AAA | Secure | Jun 26 1989 |
| Moody’s | Aaa | Secure | Nov 10 1977 |
| S&P | AAA | N/A | Oct 27 1977 |
