2013 | 2012 | 2011 | ||||||||||
Balance at January 1 | $ | 13,271 | $ | 12,767 | $ | 12,488 | ||||||
Liabilities incurred | 59 | 133 | 62 | |||||||||
Liabilities settled | (907 | ) | (966 | ) | (1,316 | ) | ||||||
Accretion expense | 627 | 629 | 628 | |||||||||
Revisions in estimated cash flows | 1,248 | 708 | 905 | |||||||||
Balance at December 31 | $ | 14,298 | $ | 13,271 | $ | 12,767 | ||||||