California Code, Insurance Code - INS § 11663

As between insurers of general and special employers, one which insures the liability of the general employer is liable for the entire cost of compensation payable on account of injury occurring in the course of and arising out of general and special employments unless the special employer had the employee on his or her payroll at the time of injury, in which case the insurer of the special employer is solely liable. For the purposes of this section, a self-insured or lawfully uninsured employer is deemed and treated as an insurer of his or her workers' compensation liability.