The following provides further details on the balance sheet accounts of accounts receivable, net; contract assets; and contract liabilities. See Note 2, Significant Accounting Policies and Estimates, for further information on our policies related to these balance sheet accounts, as well as our revenue recognition policies.



	January 25, 2020			January 26, 2019
Trade accounts receivable	$	355,805		$	331,903
Unbilled accounts receivable	453,353			283,463
Retainage	12,669			10,831
Total	821,827			626,197
Less: allowance for doubtful accounts	(4,582		)	(939		)
Accounts receivable, net	$	817,245		$	625,258

As of January 26, 2019, accounts receivable of $24.8 million from Windstream was classified as non-current in other assets and is net of the related allowance for doubtful accounts. As of January 25, 2020, all of Windstream’s accounts receivable was classified as current. See Note 7, Other Current Assets and Other Assets, for further information on our non-current accounts receivable, net.

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):



	January 25, 2020			January 26, 2019
Allowance for doubtful accounts at beginning of period	$	17,702		$	998
Provision for bad debt (recovery)	(6,540		)	16,677
Amounts recovered (charged) against the allowance	(6,580		)	27
Allowance for doubtful accounts at end of period	$	4,582		$	17,702



	January 25, 2020		January 26, 2019
Contract assets	$	253,005	$	215,849
Contract liabilities	16,332		15,125
Contract assets, net	$	236,673	$	200,724

The increase in contract assets, net, in fiscal 2020 from fiscal 2019 primarily resulted from services performed under contracts consisting of multiple tasks which will be billed as the tasks are completed. There were no other significant changes in contract assets during the period. During fiscal 2020, we performed services and recognized revenue related to all but an immaterial amount of our contract liabilities that existed at January 26, 2019. See Note 7, Other Current Assets and Other Assets, for information on our long-term contract assets.

Customers whose combined amounts of accounts receivable and contract assets, net exceeded 10% of total combined accounts receivable and contract assets, net as of January 25, 2020 or January 26, 2019 were as follows (dollars in millions):



	January 25, 2020			January 26, 2019
	Amount		% of Total	Amount		% of Total
Verizon Communications Inc.	$	440.2	41.8%	$	298.4	36.2%
CenturyLink, Inc.	$	175.8	16.7%	$	147.2	17.9%
Comcast Corporation	$	114.0	10.8%	$	127.2	15.4%
AT&T Inc.	$	97.2	9.2%	$	90.6	11.0%

We believe that none of the customers above were experiencing financial difficulties that would materially impact the collectability of our total accounts receivable and contract assets, net, as of January 25, 2020 or January 26, 2019.