as Fuel Inventory. Fuel Inventory increased during 2012 as a result of the start up of refinery operations in the September quarter and the transition of our fuel supply program. The increase in Fuel Inventory to achieve normal operating levels at our refinery was offset by a similar increase in accounts payable, and therefore, did not have a significant impact on operating cash flows during 2012. The remaining $180 million increase in Fuel Inventory was a result of the transition in Delta's fuel supply program.  

 

As part of our fuel hedging program, we may be required to pay hedge margin to counterparties when our portfolio is in a loss position. Conversely, if our portfolio with counterparties is in a gain position, we may receive hedge margin. Our future cash flows are impacted depending upon the nature of our derivative contracts and the market price of the commodities underlying our derivative contracts  

 

Timing of SkyMiles Sales. In December 2011, we amended our American Express agreements and agreed to sell $675 million of unrestricted 

SkyMiles to American Express in each December from 2011 through 2014. Under the December 2011 amendment, American Express purchased $675 million of unrestricted SkyMiles in each of 2013, 2012 and 2011. In 2011, this operating cash flow was largely offset by working capital changes. We anticipate American Express will make an additional purchase of $675 million of unrestricted SkyMiles in 2014.  

 

In 2008, we entered into a multi-year extension of our American Express agreements and received $1.0 billion from American Express for an advance purchase of restricted SkyMiles. The agreement, as modified, provided that our obligations with respect to the advance purchase would be satisfied as American Express uses the purchased miles over a specified future period (“SkyMiles Usage Period”). During the SkyMiles Usage Period, which commenced in December 2011, American Express utilized SkyMiles valued at $333 million annually over three years beginning 2012 instead of paying cash to Delta for SkyMiles used. This draw down of $333 million in SkyMiles reduced cash flows from operations in 2012 compared to 2011. In December 2013, we and American Express amended this agreement to allow American Express to use these SkyMiles immediately and without restriction.  

 

Pension Contributions. We sponsor defined benefit pension plans for eligible employees and retirees. These plans are closed to new entrants and are frozen for future benefit accruals. Our funding obligations for these plans are governed by the Employee Retirement Income Security Act, as modified by the The Pension Protection Act of 2006. We contributed $914 million in 2013, including $250 million above the minimum funding levels, $697 million in 2012 and $598 million in 2011 to our defined benefit pension plans. We estimate the funding under these plans will total approximately $925 million in 2014, including $250 million above the minimum funding requirements.  

 

Undrawn Lines of Credit  

 

We have available $1.9 billion in undrawn lines of credit. As described in Note 8 of the Notes to the Consolidated Financial Statements, we have an