6. Other Income
Other income included the following for the years ended December 31:
| Millions | 2013 [a] | 2012 | 2011 | ||||
| Rental income | $ | 106 | $ | 83 | $ | 80 | |
| Net gain on non-operating asset dispositions | 32 | 29 | 43 | ||||
| Interest income | 4 | 3 | 3 | ||||
| Early extinguishment of debt | (1) | (6) | (5) | ||||
| Non-operating environmental costs and other | (13) | (1) | (9) | ||||
| Total | $ | 128 | $ | 108 | $ | 112 | |
| [a] Rental income includes $17 million related to a land lease contract settlement. | |||||||