CONTINGENCIES, COMMITMENTS AND RESTRICTIONS ON THE DISTRIBUTION OF PROFITS (Shareholder claims, Mexican income tax adjustment & Argentine tax claim) (Details) - USD ($)
shares in Millions, $ in Millions

Dec. 31, 2017

Apr. 14, 2015

Mar. 31, 2017

Jun. 28, 2016

Comissão de Valores Mobiliários | USINAS SIDEÚRGICAS DE MINAS GERAIS S.A.

Disclosure of contingent liabilities [line items]

Number of shares exceeding threshold for tender offer requirement

5.2

Domestic Tax Authority | Mexican Tax Authority

Disclosure of contingent liabilities [line items]

Tax contingent liability

$ 58.4

Tax contingent liability | Domestic Tax Authority | Mexican Tax Authority

Disclosure of contingent liabilities [line items]

Liability from settlement with taxing authority

$ 15.9

Tax contingent liability | Domestic Tax Authority | Argentina Tax Authority

Disclosure of contingent liabilities [line items]

Liability from settlement with taxing authority including interest

$ 12.8