| Accounts Receivable |
Accounts Receivable, Contract Assets, and Contract Liabilities The following provides further details on the balance sheet accounts of accounts receivable, net; contract assets; and contract liabilities.
Accounts Receivable Accounts receivable, net, classified as current, consisted of the following (dollars in thousands): | | | | | | | | | | | | | April 25, 2020 | | January 25, 2020 | | Trade accounts receivable | $ | 384,383 | | | $ | 355,805 | | | Unbilled accounts receivable | 479,706 | | | 453,353 | | | Retainage | 11,868 | | | 12,669 | | | Total | 875,957 | | | 821,827 | | | Less: allowance for doubtful accounts | (5,166) | | | (4,582) | | | Accounts receivable, net | $ | 870,791 | | | $ | 817,245 | |
We maintain an allowance for doubtful accounts for estimated losses on uncollected balances. Approximately $16.8 million of the allowance for doubtful accounts as of January 26, 2019 was classified as non-current. The allowance for doubtful accounts changed as follows (dollars in thousands): | | | | | | | | | | | | | For the Three Months Ended | | | | April 25, 2020 | | April 27, 2019 | | Cumulative effect from implementation of ASU 2016-13 | $ | 471 | | | $ | — | | | Allowance for doubtful accounts at beginning of period | 4,582 | | | 17,702 | | | Provision for bad debt (recovery) | 118 | | | (10,311) | | | Amounts recovered (charged) against the allowance | (5) | | | (6,019) | | | Allowance for doubtful accounts at end of period | $ | 5,166 | | | $ | 1,372 | |
Contract Assets and Contract Liabilities
Net contract assets consisted of the following (dollars in thousands): | | | | | | | | | | | | | April 25, 2020 | | January 25, 2020 | | Contract assets | $ | 275,001 | | | $ | 253,005 | | | Contract liabilities | 24,566 | | | 16,332 | | | Contract assets, net | $ | 250,435 | | | $ | 236,673 | |
Net contract assets were $250.4 million and $236.7 million as of April 25, 2020 and January 25, 2020, respectively. The increase primarily resulted from services performed under contracts consisting of multiple tasks which will be billed as the tasks are completed. During the three months ended April 25, 2020, we performed services and recognized $8.5 million of contract revenues related to contract liabilities that existed at January 25, 2020. See Note 6, Other Current Assets and Other Assets, for information on our long-term contract assets.
Customer Credit Concentration
Customers whose combined amounts of accounts receivable and contract assets, net, exceeded 10% of total combined accounts receivable and contract assets, net, as of April 25, 2020 or January 25, 2020 were as follows (dollars in millions): | | | | | | | | | | | | | | | | | | | | | | | | | April 25, 2020 | | | | January 25, 2020 | | | | Amount | | % of Total | | Amount | | % of Total | | Verizon Communications Inc. | $ | 471.2 | | | 41.9% | | $ | 440.2 | | | 41.8% | | CenturyLink, Inc. | $ | 204.6 | | | 18.2% | | $ | 175.8 | | | 16.7% | | Comcast Corporation | $ | 130.0 | | | 11.6% | | $ | 114.0 | | | 10.8% | | | | | | | | |
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