According to the 2013 annual report, “The decrease in the cost of coal sales was primarily due to decreases in supply and maintenance expenses, including diesel fuel and explosives, decrease of expenses related to the merger, the decrease in expenses for the purchase of coal and the decrease in expenses for labor and social security mainly linked to the reductions in production and periods of inactivity of the mines implemented during 2012 and in the first half of 2013, decreases in credit adjustments related to changes in estimates for asset retirement obligations and other cost control measures, and decreases in sales-related costs and other variable costs associated with decreases in metallurgical and steam coal revenues and to the decrease in steam coal production, partially offset by increased expenses for regulatory issues” (p. 66). On January 31, 2011, Alpha Natural Resources acquired Massey Energy. Amortization expenses for acquired intangible assets increased approximately $75.4 million during 2013 compared to the prior year. This increase in amortization expense for acquired intangible assets is largely due to decreased amortization of below-market contracts absorbed through the Massey acquisition. This is mainly due to the completion of shipments under a large number of Alpha contracts