Simons, the firm’s chairman, is no longer the day-to-day manager. The 77-year-old has a net worth of $15.5 billion, according to the Bloomberg Billionaires Index. Medallion is almost entirely owned by Renaissance’s employees, their families and a few former employees.

“There shouldn’t be two sets of rules –- one for sophisticated organizations leveraging a complex tax system to their benefit and another for middle-class Americans trying to save for retirement,” said Senator Ron Wyden of Oregon, the top Democrat on the Finance Committee who has been probing large retirement accounts. “This is an issue of fairness, and taxpayers end up paying the price.”

It took Renaissance several years to push the changes through. Public filings tell the story.

Ending 401(k) Plan

First, in 2010, the East Setauket, New York-based firm terminated its 401(k) plan -- itself a rare step. That shifted employees’ account balances into IRAs.

The timing was no coincidence. That year, the government ended a policy preventing people earning more than $100,000 a year from converting standard IRAs into Roth IRAs by paying taxes. With standard IRAs, people invest pretax dollars and withdrawals are taxed as ordinary income. Roth IRAs, by contrast, are funded with after-tax dollars and investment gains are tax-free.

As Renaissance ended its 401(k), it approached the Labor Department for an exemption from rules prohibiting employees from investing retirement money in Medallion.

The firm’s lawyers, led by Gary Howell of Katten Muchin Rosenman LLP, argued that Medallion was outperforming the everyday mutual funds available under its old 401(k), which was administered by Fidelity Investments.

‘Exclusive Reason’

“The sole and exclusive reason why Renaissance has gone through the effort and expense to seek this exemption,” Howell wrote to the Labor Department in 2011, “is to provide a benefit to its employees that other firms do not.”

After questioning that yielded a foot-high stack of public records, the Labor Department granted the exemption in 2012.

As of the end of 2013, Renaissance was running an employer IRA plan that attracted $86.6 million in initial investments and had 259 active participants.