Christopher Cox, chairman of the U.S. Securities and Exchange Commission since August 2005, was not expected to be the activist reformer that his predecessor, William Donaldson, became in the wake of the collapse of Enron Corp. Unlike Donaldson, who will be remembered for initiating enforcement of the Sarbanes-Oxley Act and aggressively penalizing violators of trading rules, Cox has distinguished himself by fighting losing battles, such as those to require independent-director majorities on mutual fund boards and registration of hedge fund advisers.

But Cox has adopted one cause that could equal in significance, and perhaps outlast, any single financial policy prescription of the Bush years. And it’s not even a regulation — at least not yet. It’s XBRL, and Cox stumps for it as if he were running for office (as he did successfully nine times when he represented a Southern California district in the U.S. House of Representatives)....