focused on higher performing markets such as Mexico and the Caribbean, while we have reduced capacity in markets challenged by weakening currencies and slowing economies such as Brazil.

During the December 2015 quarter, we announced a plan to acquire additional capital stock of Grupo Aeroméxico through a cash tender offer. Following the completion of the tender offer, when combined with our existing share ownership, we expect to own up to 49% of Grupo Aeroméxico's outstanding shares. We have submitted an application for an antitrust immunized joint venture with Aeroméxico, which we expect will be approved in 2016, and is intended to allow us to benefit from an improved level of coordination.

Also, during 2015, we acquired additional preferred shares of GOL's parent company for $50 million, increasing our ownership to 9.5% of its outstanding capital stock, as part of our long-term strategy to strengthen the opportunity we see in Brazil. In conjunction with our investment, we and GOL agreed to extend our existing commercial agreements.

Cost Productivity

We continue to focus on maintaining the rate of non-fuel unit cost growth at less than 2% annually. An important component of this effort relates to maximizing the benefits of scale throughout the network. Domestic fleet upgauging allows us to increase capacity on fewer departures, thereby improving our unit cost performance. In addition, we are leveraging our expertise in supply chain and aircraft maintenance to improve our productivity. Finally, we are investing in technology projects to improve the customer experience in a cost-efficient manner.

The Delta pilots collective bargaining agreement became amendable as of December 31, 2015. We are in discussions with representatives of the Delta pilots to amend the collective bargaining agreement. Our employees, including pilots, are an important part of Delta’s success and the underpinning of the quality and operational reliability that has become part of Delta’s brand. We expect to continue to pay our employees industry leading total compensation that will enable Delta’s continued success.

Prudent Deployment of Cash Flows

As part of our long-term plan, we target $7-8 billion in annual operating cash flow and $4-5 billion in annual free cash flow. We use approximately 50% of our operating cash flow to reinvest in the business for long-term growth opportunities. The remaining free cash flow is used to further strengthen the balance sheet by reducing debt levels, make incremental pension contributions and return capital to shareholders through dividends and share repurchases.

Strengthened Balance Sheet

A key tenet of our strategy is to operate the company with investment grade financial metrics. Since beginning our balance sheet improvement strategy in 2009, we have reduced our adjusted net debt by over $10 billion, which has significantly reduced our annual interest expense. In addition, we have a significant net underfunded pension liability. We have been making pension contributions above the minimum amount required, with a goal of achieving 80% funded status for the pension plan by 2020. B ecause of statutory pension funding relief