A document filed with the SEC provides explicit details of the eight-month long sale of Virgin Mobile USA (NYSE: VM) to Sprint (NYSE: S) Nextel. The day-to-day account reveals that Virgin Mobile was able to increase the sale price through negotiations with more than one bidder, however, it failed to garner as much money as it initially sought.

The document was filed by Sprint Nextel and is seeking shareholder approval of the acquisition. It says Virgin Mobile USA was initially seeking a price of $6.37 a share, and that a mystery bidder called “Company X” was willing to offer prices roughly ranging between $4.27 and $5 a share during the negotiations. Meanwhile, Sprint Nextel felt strongly about its $5 a share offer. In the end of the process, which lasted from Nov. 4 to July 28, the companies announced that Sprint will pay $5.50 a share for a total of $483 million.

The documents never reveal the identity of “Company X,” but the investment banker says in the record that the bidder would likely be interested “based on its perceived financial strength, similar business model and past management dialogue with Virgin Mobile USA.” To be sure, that may include a range of companies, but perhaps a good guess is TracFone Wireless, another prepaid provider with 11 million users that is a subsidiary of America Movil.

As negotiations picked up in July, the series of events indicates that while Company X was willing to budge on price, it was dragging its heals on fulfilling certain conditions. On July 24, the document says “Company X informed Virgin Mobile USA and its financial advisors that it was withdrawing its offer,” but no other details were provided.