Ariz. Rev. Stat. § 20-545
An insurer may invest in preferred, guaranteed or preference stocks or shares of any solvent institution created or existing under the laws of the United States or of any state, district or territory of the United States if those preferred, preference or guaranteed stocks or shares have received an investment grade rating as approved by the director. The director shall by order after a hearing prescribe the investment grade ratings and the recognized rating agencies for the purposes of this section. Preferred, guaranteed or preference stocks which have not received an investment grade rating may be eligible for investment if the issuer has issued comparable preferred, guaranteed or preference stocks which have received an investment grade rating approved by the director. The director shall prescribe standards by order after a hearing by which preferred, guaranteed or preference stocks which have not received an investment grade rating may be eligible for investment.
A.R.S. § 20-545