| During 2013 and 2012, our overall asset retirement obligation changed as follows: | ||||
| Millions of Dollars | ||||
| 2013 | 2012 | |||
| Balance at January 1 | $ | 9,164 | 8,920 | |
| Accretion of discount | 434 | 412 | ||
| New obligations | 410 | 315 | ||
| Changes in estimates of existing obligations | 707 | 543 | ||
| Spending on existing obligations | (298) | (319) | ||
| Property dispositions | (163) | (607) | ||
| Foreign currency translation | (178) | 281 | ||
| Separation of Downstream business | - | (381) | ||
| Balance at December 31 | $ | 10,076 | 9,164 | |