| At December 31, 2013 | At December 31, 2012 | ||||||||||
| Convertible Senior Notes Due | Convertible Senior Notes Due | ||||||||||
| 2014 | 2017 | 2014 | 2017 | ||||||||
| Additional paid-in capital | $ | 97 | $ | 123 | $ | 97 | $ | 123 | |||
| Principal amount | $ | 575 | $ | 575 | $ | 575 | $ | 575 | |||
| Unamortized debt discount | (14) | (83) | (40) | (104) | |||||||
| Net carrying amount | $ | 561 | $ | 492 | $ | 535 | $ | 471 | |||