This excerpt taken from the WBD 20-F filed Jun 18, 2009.
Trade Receivables and Allowance for Doubtful Accounts
Trade receivables are stated at their net realizable value which approximates their fair value. The Company provides an allowance for doubtful accounts based on managements periodic review of accounts, including the delinquency of account balances. Delinquency status is based on contractual terms. The Company evaluates the collectability of its receivables at least quarterly, based upon various factors, including the financial condition and payment history of major customers, an overall review of collections experience of other accounts and economic factors or events expected to affect the Companys future collections.
Trade receivables are written-off when evidence exists that they will not be collectible. The Company generally does not require collateral from its credit customers.