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.

	June 30,			December 31,
	2019			2018
Billed and billable	$	133,443		$	176,848
Less: allowance for doubtful accounts		(4,299	)		(5,489	)
Accounts receivable, net	$	129,144		$	171,359

The Company maintains an allowance for doubtful accounts, which represents its best estimate of probable losses and sales allowances. The Company evaluates the adequacy of its allowance for doubtful accounts by considering the aging of receivable balances, historical experience, and other currently available information. The Company’s write-offs and sales allowances were not material for the three months ended June 30, 2019. For the six months ended June 30, 2019, the Company’s write-offs and sales allowances totaled $0.8 million. For the three and six months ended June 30, 2018, the Company’s write-offs and sales allowances totaled $0.8 million and $0.9 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. During the three and six months ended June 30, 2019 and 2018, 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 $0.8 million and $0.8 million in accrued sales and usage-based royalty revenue as of June 30, 2019 and December 31, 2018, respectively.

Contract liabilities are amounts received or due from customers in advance of the Company transferring the products or services to the customer. Revenue is subsequently recognized in the period(s) in which control of the products 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 products or services are expected to be transferred to the customer within the next year.

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

	June 30,		December 31,
	2019		2018
Current:
Deferred product licenses revenue	$	633	$	1,768
Deferred subscription services revenue		15,393		13,508
Deferred product support revenue		152,866		152,501
Deferred other services revenue		7,894		8,763
Total current deferred revenue and advance payments	$	176,786	$	176,540

Non-current:
Deferred product licenses revenue	$	405	$	542
Deferred subscription services revenue		116		2,384
Deferred product support revenue		3,083		3,091
Deferred other services revenue		571		452
Total non-current deferred revenue and advance payments	$	4,175	$	6,469

During the three and six months ended June 30, 2019, the Company recognized revenues of $51.0 million and $118.0 million, respectively, from amounts included in the total deferred revenue and advance payments balances at the beginning of 2019. During the three and six months ended June 30, 2018, the Company recognized revenues of $56.8 million and $131.3 million, respectively, from amounts included in the total deferred revenue and advance payments balances at the beginning of 2018. For the three and six months ended June 30, 2019 and 2018, there were no significant changes in the timing of revenue recognition on the Company’s deferred balances.

As of June 30, 2019, the Company had an aggregate transaction price of $181.0 million allocated to unsatisfied performance obligations related to product support, subscription services, other services, and product licenses contracts. The Company expects to recognize $176.8 million within the next 12 months and $4.2 million thereafter.