11	TRADE ACCOUNTS RECEIVABLE, NET

At December 31 this account comprises:


	2016					2017
	Current			Non-current		Current			Non-current
Invoices receivable		903,878			652,939		459,721			819,699
Collection rights		150,065			14,580		546,351			28,891

		1,053,943			667,519		1,006,072			848,590
Impairment of receivables		(22,673	)		—		(13,348	)		—

		1,031,270			667,519		992,724			848,590

Invoices receivable are related to estimated percentages of completion approved by customers.

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 Ferrovías S.A.

At December 31, 2017 the fair value of non-current accounts receivable amounted to S/835 million (S/606 million at December 31, 2016), which was calculated under the discounted cash flows method, using rates of 6.33% (7.14% at December 31, 2016).

At December 31, 2017, collection rights primarily relate to GyM Ferrovías S.A., CAM Holding S.p.A., Concar S.A., Survial S.A, CAM Servicios del Perú S.A. and GMI S.A.,amounting to S/355 million, S/85 million, S/53 million, S/31 million, S/21 million and S/20 million, respectively. (GyM Ferrovías S.A., GMD S.A., GMI S.A., Concar S.A., CAM Servicios del Perú S.A. y Survial for S/68 million, S/49 million, S/19 million, S/13 million, S/12 million and S/11 million, respectively, in 2016).

Aging of trade accounts receivable is as follows:


	At December 31,
	2016		2017
Current		1,396,040		1,575,709
Past due up to 30 days		103,617		118,158
Past due from 31 days up to 180 days		113,825		141,121
Past due from 181 days up to 360 days		29,506		1,962
Past due over 360 days		78,474		17,712

		1,721,462		1,854,662

At December 31, 2017, for trade accounts receivables with maturity greater than 31 days for S/160.8 million (S/221.8 million in 2016), the Group recognized impairment of S/0.7 million (S/3.1 million in 2016) in the consolidated income statement (Note 27).

The maximum exposure to credit risk at the reporting date is the carrying amount of the accounts receivable and of unbilled work in progress (Note 12).