2013 | 2012 | ||||||
(Millions) | |||||||
Balance, January 1 | $ | 321 | $ | 289 | |||
Liabilities incurred during the period | 14 | 19 | |||||
Liabilities settled during the period | (11 | ) | (7 | ) | |||
Estimate revisions | 17 | (1 | ) | ||||
Accretion expense(a) | 23 | 21 | |||||
Balance, December 31 | $ | 364 | $ | 321 | |||
Amount reflected as current | $ | 6 | $ | 5 | |||
(a) | Accretion expense is included in lease and facility operating expense on the Consolidated Statements of Operations. |