They buy a few from Kenworth when production at Freightliner and Peterbilt is bogged down to meet their acquisition goals. That's it.
They bought a few T660's a few years ago - they're all rotated out of the fleet. I've heard a rumor they may be buying a few more this year, but I've yet to see one. That doesn't surprise me though, since many carriers are finally putting in large orders for tractors now.