CBS comedy “The Big Bang Theory” retained nearly all of its young-adult audience in its first showdown with Fox heavyweight “American Idol” on Thursday, which also saw NBC’s “The Office” surge to a season high as part of the net’s new six-night comedy lineup.

Fox won the night, according to preliminary national estimates from Nielsen, logging its highest-rated Thursday with a regular lineup since February 1995 behind “American Idol” (7.8 rating/21 share in adults 18-49, 22.9 million viewers overall) and the timeslot debut of “Bones” (3.6/9 in 18-49, 10.9 million viewers overall).

“Idol,” making its regular-night debut, dominated the 8 o’clock hour despite falling 15% in 18-49 and 9% in total viewers from its opening hour of Wednesday night; it did soar 22% in the demo above the show’s Thursday average of last season when it aired three special episodes on the night in late winter. “Bones” followed with its best scores in about a year, placing second in most categories.

Elsewhere, CBS leadoff hitter “The Big Bang Theory” (4.2/12 in 18-49, 13.6 million viewers overall) dropped only slightly vs. its recent average to place a strong second to “Idol” at 8 p.m., but the net saw a steep dropoff at 8:30 p.m. for “Bleep My Dad Says” (2.7/7 in 18-49, 10.3 million viewers overall), which figured to be more vulnerable to “Idol.” The Eye followed the comedies with “CSI” (3.1/8 in 18-49, 14.3 million viewers overall), which ran third in 18-49 but first in total viewers, and “The Mentalist” (2.8/8 in 18-49, 14.7 million viewers overall), which led the broadcast pack at 10.

NBC debuted its six-comedy night to fairly good results, with “Community” holding up well at 8 and by matching its season high for a regular-slot telecast (2.2/6 in 18-49, 4.7 million viewers overall) and “Perfect Couples” doing OK in its bow (2.1/6 in 18-49, 4.2 million viewers overall), considering it was taking on “American Idol.” At 9, “The Office” seemed to benefit from the “Idol” overflow at Fox, hitting a season high (4.5/12 in 18-49, 8.3 million viewers overall) and helping “Parks and Recreation” (3.2/8 in 18-49, 6.2 million viewers overall) establish a series high as it kicked off its season. Closing out the night, “30 Rock” (2.7/7 in 18-49, 5.3 million viewers overall) earned the net’s top non-sports 18-49 rating in the 10 o’clock half-hour since April, and “Outsourced” (1.8/5 in 18-49, 4.0 million viewers overall) did the best since at 10:30 p.m. since May despite losing 2 share points from its lead-in.

ABC’s “Wipeout” (2.6/7 in 18-49, 8.3 million viewers overall) dropped off some opposite “American Idol” but still provided the net with huge gains over this time last year. It was followed by repeats of “Grey’s Anatomy” (1.6/4 in 18-49, 4.7 million viewers overall) and “Private Practice” (1.0/3 in 18-49, 3.3 million viewers overall).

Preliminary 18-49 averages for the night: Fox, 5.7/15; CBS, 3.1/8; NBC, 2.8/7; ABC, 1.7/5; Univision, 1.6/4; CW, 0.5/1.

In total viewers: Fox, 16.8 million; CBS, 13.6 million; ABC and NBC, 5.4 million; Univision, 3.9 million; CW, 1.3 million.