Moody's Investors Service – the credit rating arm of Moody's Corp. ( MCO ) – has placed the ratings of Zions Bancorp ( ZION ) and its affiliates on review for a likely upgrade. The main reason behind the possible rise in rating is the company's plummeting asset concentrations. Currently, Zions' subordinated debt is rated 'B1.' Moreover, its main operating bank, Zions First National Bank, has a financial strength rating of 'D+', which reflects baseline credit assessment of 'ba1.' The bank's financial strength rating of 'D+' was affirmed, but its baseline credit assessment of 'ba1' will be appraised for promotion.