|Millions of euro|
|at Dec. 31, 2020||at Dec. 31, 2019||Change|
|Amounts due to customers||1,481||1,670||(189)||-11.3%|
|Amounts due to institutional market operators||4,012||4,507||(495)||-11.0%|
|Amounts due to employees||438||496||(58)||-11.7%|
|Other tax liabilities||886||1,082||(196)||-18.1%|
|Amounts due to social security institutions||207||212||(5)||-2.4%|
|Put options granted to non-controlling shareholders||1||3||(2)||-66.7%|
|Current accrued expenses and deferred income||346||372||(26)||-7.0%|
Amounts due to customers include €822 million (€880 million at December 31, 2019) in security deposits related to amounts received from customers in Italy as part of electricity and gas supply contracts. Following the finalization of the contract, deposits for electricity sales, the use of which is not restricted in any way, are classified as current liabilities given that the Parent does not have an unconditional right to defer repayment beyond 12 months.
Amounts due to institutional market operators include liabilities arising from the application of equalization mechanisms to electricity purchases on the Italian market amounting to €2,444 million (€3,064 million at December 31, 2019), on the Spanish market amounting to €1,538 million (€1,267 million at December 31, 2019) and on the Latin American market amounting to €30 million (€176 million at December 31, 2019).
Contingent consideration mainly regards a number of equity investments held by the Group in North America, the fair value of which was determined on the basis of the contractual conditions in the agreements between the parties.
Other mainly regards the liabilities of some Brazilian companies to the national electricity agency ANEEL (Regulatory Resolution no, 885/2020 of June 23) in respect of loans granted to distribution companies in order to provide liquidity to them and minimize the effects of the pandemic.
The decline in other is mainly attributable to the effect of the recognition in 2019 of the debt of €358 million associated with the purchase through financial intermediaries (with share swaps) of additional shares in Enel Américas and Enel Chile, compounded by the impact of the reduction in 2020 of liabilities for expired commodity derivatives, which were registered mainly in Italy and Spain. These effects were partially offset by an increase in liabilities for tax partnerships (€87 million) posted by renewables companies in North America in the amount of €181 million as a result of the entry of new plants into service.