TD Ameritrade ( AMTD) announced Wednesday it would buy back 34 million shares from founder Joe Ricketts' family, which will use the proceeds to help finance its purchase of the Chicago Cubs.The online brokerage firm will buy the Ricketts' stock for $11.85 per share as part of its existing buyback programs, the company said in a statement. These buyback programs include the repurchase of shares it plans to issue in its proposed acquisition of thinkorswim Group ( SWIM). "This transaction with our founder will allow us to complete our existing buyback plans with immediate accretion for all of our shareholders," said Ameritrade CEO Fred Tomczyk. The Ricketts family, Ameritrade's second largest shareholder, will continue to own 17.7% of the company's shares and hold two seats on its board. Shares of Ameritrade, which competes with such companies as Charles Schwab ( SCHB) and E*Trade Financial Corp. ( ETFC), were down 0.8% to $12.47 in recent trading.