FINANCIAL RISK MANAGEMENT - Liquidity risk (Details) - USD ($) $ in Millions | Dec. 31, 2017 | Dec. 31, 2016 |
Disclosure of maturity analysis for financial assets held for managing liquidity risk [line items] | | |
Financial liabilities | $ 8,892 | |
Derivative financial liabilities | 62 | $ 7 |
Non-derivative, Contractual cash flows | 12,101 | |
Foreign exchange derivative contracts [member] | | |
Disclosure of maturity analysis for financial assets held for managing liquidity risk [line items] | | |
Derivative financial liabilities | 10 | |
Derivative, Contractual cash flows | 10 | |
Natural gas derivative contracts [member] | | |
Disclosure of maturity analysis for financial assets held for managing liquidity risk [line items] | | |
Derivative financial liabilities | 80 | |
Derivative, Contractual cash flows | 80 | |
Short-term debt [member] | | |
Disclosure of maturity analysis for financial assets held for managing liquidity risk [line items] | | |
Financial liabilities | 867 | |
Non-derivative, Contractual cash flows | 867 | |
Accounts payable [member] | | |
Disclosure of maturity analysis for financial assets held for managing liquidity risk [line items] | | |
Financial liabilities | 3,495 | |
Non-derivative, Contractual cash flows | 3,495 | |
Current portion of long-term debt [member] | | |
Disclosure of maturity analysis for financial assets held for managing liquidity risk [line items] | | |
Financial liabilities | 11 | |
Non-derivative, Contractual cash flows | 11 | |
Long-term debt [member] | | |
Disclosure of maturity analysis for financial assets held for managing liquidity risk [line items] | | |
Financial liabilities | 4,397 | |
Non-derivative, Contractual cash flows | 7,606 | |
Other liabilities [member] | | |
Disclosure of maturity analysis for financial assets held for managing liquidity risk [line items] | | |
Financial liabilities | 32 | |
Non-derivative, Contractual cash flows | 32 | |
Less than one year [member] | | |
Disclosure of maturity analysis for financial assets held for managing liquidity risk [line items] | | |
Non-derivative, Contractual cash flows | 4,654 | |
Less than one year [member] | Foreign exchange derivative contracts [member] | | |
Disclosure of maturity analysis for financial assets held for managing liquidity risk [line items] | | |
Derivative, Contractual cash flows | 10 | |
Less than one year [member] | Natural gas derivative contracts [member] | | |
Disclosure of maturity analysis for financial assets held for managing liquidity risk [line items] | | |
Derivative, Contractual cash flows | 52 | |
Less than one year [member] | Short-term debt [member] | | |
Disclosure of maturity analysis for financial assets held for managing liquidity risk [line items] | | |
Non-derivative, Contractual cash flows | 867 | |
Less than one year [member] | Accounts payable [member] | | |
Disclosure of maturity analysis for financial assets held for managing liquidity risk [line items] | | |
Non-derivative, Contractual cash flows | 3,495 | |
Less than one year [member] | Current portion of long-term debt [member] | | |
Disclosure of maturity analysis for financial assets held for managing liquidity risk [line items] | | |
Non-derivative, Contractual cash flows | 11 | |
Less than one year [member] | Long-term debt [member] | | |
Disclosure of maturity analysis for financial assets held for managing liquidity risk [line items] | | |
Non-derivative, Contractual cash flows | 219 | |
Less than one year [member] | Other liabilities [member] | | |
Disclosure of maturity analysis for financial assets held for managing liquidity risk [line items] | | |
Non-derivative, Contractual cash flows | 0 | |
One to three years [member] | | |
Disclosure of maturity analysis for financial assets held for managing liquidity risk [line items] | | |
Non-derivative, Contractual cash flows | 915 | |
One to three years [member] | Foreign exchange derivative contracts [member] | | |
Disclosure of maturity analysis for financial assets held for managing liquidity risk [line items] | | |
Derivative, Contractual cash flows | 0 | |
One to three years [member] | Natural gas derivative contracts [member] | | |
Disclosure of maturity analysis for financial assets held for managing liquidity risk [line items] | | |
Derivative, Contractual cash flows | 28 | |
One to three years [member] | Short-term debt [member] | | |
Disclosure of maturity analysis for financial assets held for managing liquidity risk [line items] | | |
Non-derivative, Contractual cash flows | 0 | |
One to three years [member] | Accounts payable [member] | | |
Disclosure of maturity analysis for financial assets held for managing liquidity risk [line items] | | |
Non-derivative, Contractual cash flows | 0 | |
One to three years [member] | Current portion of long-term debt [member] | | |
Disclosure of maturity analysis for financial assets held for managing liquidity risk [line items] | | |
Non-derivative, Contractual cash flows | 0 | |
One to three years [member] | Long-term debt [member] | | |
Disclosure of maturity analysis for financial assets held for managing liquidity risk [line items] | | |
Non-derivative, Contractual cash flows | 868 | |
One to three years [member] | Other liabilities [member] | | |
Disclosure of maturity analysis for financial assets held for managing liquidity risk [line items] | | |
Non-derivative, Contractual cash flows | 19 | |
Four to five years [member] | | |
Disclosure of maturity analysis for financial assets held for managing liquidity risk [line items] | | |
Non-derivative, Contractual cash flows | 867 | |
Four to five years [member] | Foreign exchange derivative contracts [member] | | |
Disclosure of maturity analysis for financial assets held for managing liquidity risk [line items] | | |
Derivative, Contractual cash flows | 0 | |
Four to five years [member] | Natural gas derivative contracts [member] | | |
Disclosure of maturity analysis for financial assets held for managing liquidity risk [line items] | | |
Derivative, Contractual cash flows | 0 | |
Four to five years [member] | Short-term debt [member] | | |
Disclosure of maturity analysis for financial assets held for managing liquidity risk [line items] | | |
Non-derivative, Contractual cash flows | 0 | |
Four to five years [member] | Accounts payable [member] | | |
Disclosure of maturity analysis for financial assets held for managing liquidity risk [line items] | | |
Non-derivative, Contractual cash flows | 0 | |
Four to five years [member] | Current portion of long-term debt [member] | | |
Disclosure of maturity analysis for financial assets held for managing liquidity risk [line items] | | |
Non-derivative, Contractual cash flows | 0 | |
Four to five years [member] | Long-term debt [member] | | |
Disclosure of maturity analysis for financial assets held for managing liquidity risk [line items] | | |
Non-derivative, Contractual cash flows | 862 | |
Four to five years [member] | Other liabilities [member] | | |
Disclosure of maturity analysis for financial assets held for managing liquidity risk [line items] | | |
Non-derivative, Contractual cash flows | 5 | |
More than five years [member] | | |
Disclosure of maturity analysis for financial assets held for managing liquidity risk [line items] | | |
Non-derivative, Contractual cash flows | 5,665 | |
More than five years [member] | Foreign exchange derivative contracts [member] | | |
Disclosure of maturity analysis for financial assets held for managing liquidity risk [line items] | | |
Derivative, Contractual cash flows | 0 | |
More than five years [member] | Natural gas derivative contracts [member] | | |
Disclosure of maturity analysis for financial assets held for managing liquidity risk [line items] | | |
Derivative, Contractual cash flows | 0 | |
More than five years [member] | Short-term debt [member] | | |
Disclosure of maturity analysis for financial assets held for managing liquidity risk [line items] | | |
Non-derivative, Contractual cash flows | 0 | |
More than five years [member] | Accounts payable [member] | | |
Disclosure of maturity analysis for financial assets held for managing liquidity risk [line items] | | |
Non-derivative, Contractual cash flows | 0 | |
More than five years [member] | Current portion of long-term debt [member] | | |
Disclosure of maturity analysis for financial assets held for managing liquidity risk [line items] | | |
Non-derivative, Contractual cash flows | 0 | |
More than five years [member] | Long-term debt [member] | | |
Disclosure of maturity analysis for financial assets held for managing liquidity risk [line items] | | |
Non-derivative, Contractual cash flows | 5,657 | |
More than five years [member] | Other liabilities [member] | | |
Disclosure of maturity analysis for financial assets held for managing liquidity risk [line items] | | |
Non-derivative, Contractual cash flows | $ 8 | |