american tower corporation and subsidiaries notes to consolidated financial statements financial statements include impairment of long-lived assets ( including goodwill ) , asset retirement obligations , revenue recognition , rent expense , stock-based compensation , income taxes and accounting for business combinations . the company considers events or transactions that occur after the balance sheet date but before the financial statements are issued as additional evidence for certain estimates or to identify matters that require additional disclosure . concentrations of credit risk 2014the company is subject to concentrations of credit risk related to its cash and cash equivalents , notes receivable , accounts receivable , deferred rent asset and derivative financial instruments . the company mitigates its risk with respect to cash and cash equivalents and derivative financial instruments by maintaining its deposits and contracts at high quality financial institutions and monitoring the credit ratings of those institutions . the company derives the largest portion of its revenues , corresponding accounts receivable and the related deferred rent asset from a relatively small number of tenants in the telecommunications industry , and approximately 56% ( 56 % ) of its current year revenues are derived from four tenants . in addition , the company has concentrations of credit risk in certain geographic areas . the company mitigates its concentrations of credit risk with respect to notes and trade receivables and the related deferred rent assets by actively monitoring the credit worthiness of its borrowers and tenants . in recognizing customer revenue , the company must assess the collectibility of both the amounts billed and the portion recognized in advance of billing on a straight-line basis . this assessment takes tenant credit risk and business and industry conditions into consideration to ultimately determine the collectibility of the amounts billed . to the extent the amounts , based on management 2019s estimates , may not be collectible , recognition is deferred until such point as collectibility is determined to be reasonably assured . any amounts that were previously recognized as revenue and subsequently determined to be uncollectible are charged to bad debt expense included in selling , general , administrative and development expense in the accompanying consolidated statements of operations . accounts receivable is reported net of allowances for doubtful accounts related to estimated losses resulting from a tenant 2019s inability to make required payments and allowances for amounts invoiced whose collectibility is not reasonably assured . these allowances are generally estimated based on payment patterns , days past due and collection history , and incorporate changes in economic conditions that may not be reflected in historical trends , such as tenants in bankruptcy , liquidation or reorganization . receivables are written-off against the allowances when they are determined to be uncollectible . such determination includes analysis and consideration of the particular conditions of the account . changes in the allowances were as follows for the years ended december 31 , ( in thousands ) : .
for the four largest tenants , what is the average % (  % ) of current year revenues that each represents?
functional currency 2014the functional currency of each of the company 2019s foreign operating subsidiaries is the respective local currency , except for costa rica , where the functional currency is the u.s . dollar . all foreign currency assets and liabilities held by the subsidiaries are translated into u.s . dollars at the exchange rate in .