Historie der Langfrist-Ratings
Standard & Poor's
| 2008 - heute | A+ |
| 2007 - 2008 | AA |
| 2002 - 2007 | AA- |
| 1999 - 2002 | AA |
| 1998 - 1999 | AA+ |
| vor 1998 | AAA |
Fitch Ratings
| 2011 - heute | A+ |
| 2002 - 2011 | AA- |
| 1999 - 2002 | AA |
| 1998 - 1999 | AA+ |
| vor 1998 | AAA |
Moody's Investors Service
| 2010 - heute | Aa3 |
| 2007 - 2010 | Aa1 |
| 1999 - 2007 | Aa3 |
| 1996 - 1999 | Aa1 |
| vor 1996 | Aaa |