11	TRADE ACCOUNTS RECEIVABLES, NET

At December 31 this account comprises:


	2017					2018
	Current			Non-current		Current			Non-current
Invoice receivables		459,722			819,699		907,007			469,510
Unbilled receivables		1,069,299			87,888		113,464			550,557

		1,529,021			907,587		1,020,471			1,020,067
(-) Impairment of account receivables		(13,348	)		—		(12,643	)		—

		1,515,673			907,587		1,007,828			1,020,067

The fair value of current receivables is similar to their carrying amount since their average collection turnover is less than 60 days. These current receivables do not bear interest and have no specific guarantees.

The non-current portion of the trade accounts receivable is related to the financial asset model (Note 2.5) of subsidiary GyM Ferrovias S.A.

At December 31, 2018, the fair value of non-current accounts receivable amounted to S/1,060 million (S/835 million at December 31, 2017), which was calculated under the discounted cash flows method, using rates of 7.33% (6.33% at December 31, 2017).

Unbilled receivables are documents related to estimates for services rendered that were not billed by the Engineering and Construction segment related to estimates of the completion advance percentage. Until such revenues are billed, they are recorded in the unbilled receivables account. As of December 31, 2018, the carrying value of non-current unbilled receivables is similar to their fair value, as they were recorded using the discounted cash flow method, using a rate of 1.71%.

Rights for concessions in progress correspond to future collection rights for public service concessions that are still in the pre-operational stage.

At December 31, 2018, current and non-current unbilled receivables mainly from the following subsidiaries are as follows:


Unbilled receivables	2017		2018
GyM S.A.		581,946		14,455
GyM Ferrovias		354,763		558,179
Concar S.A.		52,508		38,770
Survial S.A.		30,647		19,138
GMI S.A.		19,699		26,622
Norvial S.A.		7,057		2,885
Cam Holding SPA		85,366		—
Others		25,201		3,972

		1,157,187		664,021

Aging of trade accounts receivable is as follows:


	2017		2018
Current		2,157,656		1,866,913
Past due up to 30 days		118,158		37,750
Past due from 31 days up to 180 days		141,120		25,854
Past due from 181 days up to 360 days		1,962		17,660
Past due over 360 days		17,712		92,361

		2,436,608		2,040,538

The Group has recognized impairment amounting to S/3.1 million (S/0.7 million in 2017 and S/3.1 in 2016) in the consolidated statement of income (Note 27). The maximum exposure to credit risk at the reporting date is the carrying amount of the accounts receivable and unbilled work in progress (Note 12).