| Company | Ranking | Outlook | Date |
|---|---|---|---|
| S&P | A+ | Secure | Jul 15 2022 |
| S&P | A+ | Destructive | Jul 16 2021 |
| Moody’s | A1 | Secure | Sep 22 2020 |
| S&P | AA- | Destructive | Jul 17 2020 |
| Moody’s | Aa2 | Beneath Evaluation | Mar 30 2020 |
| S&P | AA- | Secure | Mar 26 2020 |
| Moody’s | Aa2 | Secure | Could 26 2017 |
| Moody’s | Aa2 | Destructive | Could 14 2016 |
| Moody’s | Aa2 | Destructive Watch | Mar 04 2016 |
| S&P | AA | Secure | Jul 20 2011 |
| Moody’s | Aa2 | Secure | Aug 05 2010 |
| Moody’s | Aa2 | Destructive | Jun 09 2009 |
| Moody’s | Aa2 | Destructive Watch | Mar 19 2009 |
| Moody’s | Aa2 | Secure | Jul 25 2007 |
| S&P | AA- | Secure | Jul 16 2007 |
| Moody’s | Aa3 | Secure | Oct 04 2006 |
| Moody’s | A2 | Constructive Watch | Sep 07 2006 |
| Moody’s | A2 | Constructive | Feb 15 2006 |
| Moody’s | A2 | Secure | Could 15 2002 |
| S&P | A+ | Secure | Apr 04 2002 |
| Moody’s | Baa1 | Constructive | Nov 15 1999 |
| S&P | A | Secure | Oct 23 1997 |
| Moody’s | Baa1 | Secure | Jan 29 1996 |
