| Operating lease rental expense for the years ended December 31 was: | |||||||
| Millions of Dollars | |||||||
| 2013 | 2012 | 2011 | |||||
| Total rentals* | $ | 317 | 282 | 304 | |||
| Less: sublease rentals | (12) | (15) | (14) | ||||
| $ | 305 | 267 | 290 | ||||
| *Includes $3 million and $29 million of contingent rentals in 2012 and 2011, respectively. Contingent rentals were primarily related to drilling | |||||||
| equipment and based on usage. | |||||||