The Company invoices its customers in accordance with billing schedules established in each contract. The Company’s rights to consideration from customers are presented separately in the Company’s Consolidated Balance Sheets depending on whether those rights are conditional or unconditional.

The Company presents unconditional rights to consideration from customers within “Accounts receivable, net” in its Consolidated Balance Sheets. All of the Company’s contracts are generally non-cancellable and/or non-refundable, and therefore an unconditional right generally exists when the customer is billed or amounts are billable per the contract.

	December 31,
	2019			2018
Billed and billable	$	165,153		$	176,848
Less: allowance for doubtful accounts		(1,637	)		(5,489	)
Accounts receivable, net	$	163,516		$	171,359

The Company maintains an allowance for doubtful accounts, which represents its best estimate of sales allowances and probable credit losses. When evaluating the adequacy of its allowance for doubtful accounts, the Company factors in the aging of receivable balances, historical experience, and current information. For the years ended December 31, 2019, 2018, and 2017, the Company’s sales allowances and bad debt expense totaled $0.1 million, $1.9 million, and $2.3 million, respectively.

In contrast, rights to consideration that are subject to a condition other than the passage of time are considered contract assets and presented within “Prepaid expenses and other current assets” in the Consolidated Balance Sheets since the rights to consideration are expected to become unconditional and transfer to accounts receivable within one year. Contract assets generally consist of accrued sales and usage-based royalty revenue. In these arrangements, consideration is not billed or billable until the royalty reporting is received, generally in the subsequent quarter, at which time the contract asset transfers to accounts receivable and a true-up adjustment is recorded to revenue. These true-up adjustments are generally not material. During the years ended December 31, 2019, 2018, and 2017, there were no significant impairments to the Company’s contract assets, nor were there any significant changes in the timing of the Company’s contract assets being reclassified to accounts receivable. Contract assets included in “Prepaid expenses and other current assets” in the Consolidated Balance Sheets consisted of $1.2 million and $0.8 million in accrued sales and usage-based royalty revenue as of December 31, 2019 and 2018, respectively.

Contract liabilities are amounts received or due from customers in advance of the Company transferring the software or services to the customer. Revenue is subsequently recognized in the period(s) in which control of the software or services is transferred to the customer. The Company’s contract liabilities are presented as either current or non-current “Deferred revenue and advance payments” in the Consolidated Balance Sheets, depending on whether the software or services are expected to be transferred to the customer within the next year.

The Company’s “Accounts receivable, net” and “Deferred revenue and advance payments” balances in the Consolidated Balance Sheets include unpaid amounts related to contracts under which the Company has an enforceable right to invoice the customer for non-cancellable and/or non-refundable software and services. Changes in accounts receivable and changes in deferred revenue and advance payments are presented net of these unpaid amounts in “Operating activities” in the Consolidated Statements of Cash Flows.

Deferred revenue and advance payments (in thousands) from customers consisted of the following, as of:

	December 31,
	2019		2018
Current:
Deferred product licenses revenue	$	481	$	1,768
Deferred subscription services revenue		16,561		13,508
Deferred product support revenue		161,670		152,501
Deferred other services revenue		8,395		8,763
Total current deferred revenue and advance payments	$	187,107	$	176,540

Non-current:
Deferred product licenses revenue	$	293	$	542
Deferred subscription services revenue		97		2,384
Deferred product support revenue		3,417		3,091
Deferred other services revenue		537		452
Total non-current deferred revenue and advance payments	$	4,344	$	6,469

During the years ended December 31, 2019, 2018, and 2017, the Company recognized revenues of $174.7 million, $194.6 million, and $196.7 million, respectively, from amounts included in the total deferred revenue and advance payments balances at the beginning of the respective year. For the years ended December 31, 2019, 2018, and 2017, there were no significant changes in the timing of revenue recognition on the Company’s deferred balances.

As of December 31, 2019, the Company had an aggregate transaction price of $191.5 million allocated to unsatisfied performance obligations related to product support, subscription services, other services, and, in limited cases, product licenses contracts. The Company expects to recognize $187.1 million within the next 12 months and $4.3 million thereafter.