Over the past three years, “American Idol” has kept Fox so far ahead of the competition that its inability to program successful new shows hasn’t much mattered.

But with “Idol’s” audience slipping at the end of last season, and ratings for aging hits like “24” and “The Simpsons” sliding, Fox’s poor development has become a potentially serious liability.

It’s ready for Kevin Reilly, and Reilly has lots of work to do. For all its past edginess, Fox of late has suffered from a lack of creative direction in the shows it chooses to air and often poor execution.

Reilly officially joined Fox yesterday after weeks of speculation that he would be reunited with former FX boss and current Fox head Peter Liguori. NBC booted Reilly as entertainment president in May after the network finished fourth in adults 18-49 for the third straight year.

Reilly becomes Fox Entertainment president while Liguori moves up to entertainment chairman. Reilly will focus on development and programming, while Liguori will add oversight of Fox’s online broadcasting to his duties.

Reilly’s immediate challenge will be to turn around the network’s dismal fall performance. Fox has produced only one hit new show the past three years, the medical drama “House,” and a couple solid but unspectacular programs, “Prison Break” and “Bones.”

That has forced it to play catch-up each winter after slipping behind the other Big Four networks, though it has won the last three seasons thanks to “Idol.”

In that time, Fox’s median age shot up from the mid-30s to over 40 for the first time ever, leading the network to offer less ribald shows than in the past.

Last fall, in an attempt to program to that new, more grown-up image, Fox debuted a batch of milquetoast cop, lawyer and family sitcoms that received universally poor reviews. Only one of those programs, the Brad Garrett comedy “’Til Death,” was renewed.

This is where Reilly could prove a huge help. He excels at finding smart, edgy programs capable of attracting large audiences. At FX, he helped develop “The Shield” and “Nip/Tuck,” and at NBC he nurtured unlikely hits “The Office,” “My Name is Earl” and “Heroes.”

Though some critics say Reilly’s tastes are too quirky for broadcast, no one has argued with the quality of his programs. Even recent NBC flops like “Kidnapped,” “Friday Night Lights” and “Studio 60 on the Sunset Strip” received strong reviews.

The big problem at NBC was that the network’s audience had been so depleted by aging shows and low-quality sitcoms during the Jeff Zucker years that it was hard to attract a new audience to good shows.

That won’t be a problem at Fox. The network has a strong platform to launch new shows, with its summer lineup No. 1 among adults 18-49 and “Idol” and “House” capable of bringing big viewership to new programs.

The larger issue has been audiences did not like what they saw. But Reilly showed with “Deal or No Deal,” the older-skewing hit game show, that his successes aren’t limited to higher-brow programs alone.

Both Liguori and Reilly say they envision a programming slate with some of FX’s edge yet mainstream enough to stand out on broadcast.

Indeed, several of Fox’s upcoming programs, including “Amsterdam,” about a 400-year-old cop, “K-Ville,” about New Orleans police post-Katrina, and “The Return of Jezebel James,” a comedy from “Gilmore Girls” creator Amy Sherman-Palladino, have gotten decent buzz for their originality.

It’s in the execution, where all three shows could be shaky, that Reilly may really help by making the plots and characters more accessible to audiences, as he did with NBC’s “30 Rock” and “Office,” which showed great improvement during their first-season runs.

Reilly’s influence will also be seen at midseason. He’ll likely rescreen a number of passed-over pilots. Perhaps his biggest task will be finding a new hit comedy for Fox, as “Simpsons” stretches into its 19th year.

Whereas at NBC, this was a slow and daunting process, with Zucker often second-guessing Reilly’s decisions and never giving him untethered support, Liguori exhibits unqualified support and trust in his new No. 2’s instincts.

Also, beyond being free of Zucker, Reilly will be free of the budget restraints that hobbled his efforts to reinvigorate NBC's primetime lineup. With two straight record upfront tallies under Liguori, and so much to gain from Reilly's leadership, Fox can be expected to give its new entertainment chief whatever he asks for.