| December 31, | May 31, | |||||||||
| (in millions) | 2013 | 2013 | 2012 | |||||||
| AROs, beginning of year | $ | 658.5 | $ | 600.3 | $ | 573.1 | ||||
| Liabilities incurred | 13.6 | 38.7 | 27.8 | |||||||
| Liabilities settled | (37.0) | (73.2) | (98.4) | |||||||
| Accretion expense | 21.6 | 33.3 | 32.4 | |||||||
| Revisions in estimated cash flows | 67.2 | 59.4 | 65.4 | |||||||
| AROs, end of year | 723.9 | 658.5 | 600.3 | |||||||
| Less current portion | 86.3 | 83.5 | 87.0 | |||||||
| $ | 637.6 | $ | 575.0 | $ | 513.3 | |||||