The increase in depreciation and amortization expenses of $9.5 million was primarily attributable to assets acquired in 2009 that had a full year period of depreciation in 2010, and incremental depreciation on capital expenditures of $143.6 million that occurred during 2010.
General and administrative expenses increased $3.9 million reflecting outside services expenses.
The $49.0 million decrease in interest expense was primarily due to an overall reduction in long-term debt of $379.6 million and lower interest rates on existing debt.
The gain on mark-to-market derivative instruments for 2010 compared to a loss for 2009 was primarily due to the treatment of commodity hedges related to the Permian assets and Versado that were allocated to the Partnership through common control accounting. These allocated hedges did not qualify for hedge accounting prior to the Partnership's acquisition of the underlying assets and therefore the change in fair value of these instruments was recorded in earnings using the mark-to-market method. The use of mark-to-market accounting caused non-cash earnings volatility due to changes in the underlying commodity price indices. During 2010, the Partnership recorded mark-to-market gain, compared to 2009, when the Partnership recorded mark-to-market loss due to these changes in commodity price indices.
Targa Resources Partners - Review of Segment Performance
The following discussion of segment performance includes inter-segment revenues. The Partnership views segment operating margin as an important performance measure of the core profitability of its operations. This measure is a key component of internal financial reporting and is reviewed for consistency and trend analysis. For a discussion of operating margin, see "Targa Resources Partners -- Non-GAAP Financial Measures -- Operating Margin." Segment operating financial results and operating statistics include the effects of intersegment transactions. These intersegment transactions have been eliminated from the consolidated presentation. For all operating statistics presented, the numerator is the total volume or sales for the period and the denominator is the number of calendar days for the period.