Target's CEO is leaving his post after a hack of its systems saw 70 million customers' personal information stolen over the holidays. Gregg Steinhafel, who has been with the company for 35 years, is resigning immediately. He will be replaced, for the time being, by chief financial officer John Mulligan, the Wall Street Journal reports. Board member Roxanne Austin will temporarily replace Steinhafel as chairman. "The last several months have tested Target in unprecedented ways," Steinhafel says in a letter to the board. The outgoing CEO "held himself personally accountable (for the breach) and pledged that Target would emerge a better company," the firm says in a statement, via Forbes.