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BASIS OF PRESENTATION AND SIGNIFICANT ACCOUNTING POLICIES (Tables) | 12 Months Ended
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Dec. 31, 2017
Schedule Of Subsidiaries | The consolidated financial statements include our accounts and the accounts of our wholly owned and majority-owned subsidiaries, including the following operations at December 31, 2017:
Name |  | Location |  | Ownership Interest |  | Operation |  | Status of Operations
Northshore |  | Minnesota |  | 100.0% |  | Iron Ore |  | Active
United Taconite |  | Minnesota |  | 100.0% |  | Iron Ore |  | Active
Tilden 1 |  | Michigan |  | 100.0% |  | Iron Ore |  | Active
Empire 1 |  | Michigan |  | 100.0% |  | Iron Ore |  | Indefinitely Idled
Koolyanobbing |  | Western Australia |  | 100.0% |  | Iron Ore |  | Active
1 During 2017, our ownership interest in Tilden and Empire changed. Refer to the Noncontrolling Interests section below for additional information.
Depreciation Disclosure [Table Text Block] | Depreciation is provided over the following estimated useful lives:
Asset Class |  | Basis |  | Life
Office and information technology |  | Straight line |  | 3 to 15 Years
Buildings |  | Straight line |  | 45 Years
Mining equipment |  | Straight line/Double declining balance |  | 3 to 20 Years
Processing equipment |  | Straight line |  | 10 to 45 Years
Electric power facilities |  | Straight line |  | 10 to 45 years
Land improvements |  | Straight line |  | 20 to 45 years
Asset retirement obligation |  | Straight line |  | Life of mine
Estimated Useful Lives Of Intangible Assets Subject To Periodic Amortization On Straight Line Basis Table [Text Block] | Other intangible assets are subject to periodic amortization over their estimated useful lives as follows:
Intangible Assets |  | Basis |  | Useful Life
Permits - Asia Pacific Iron Ore |  | Units of production |  | Life of mine
Permits - USIO |  | Straight line |  | Life of mine
Reimbursements Revenue Disclosure [Table Text Block] | The following table is a summary of reimbursements in our U.S. Iron Ore operations for the years ended December 31, 2017, 2016 and 2015:
|  | (In Millions)
|  | Year Ended December 31,
|  | 2017 |  | 2016 |  | 2015
Reimbursements for: |  |  |  |  |  |
Freight |  | $ | 166.7 |  |  | $ | 106.8 |  |  | $ | 105.3 |
Venture partners’ cost |  | 54.7 |  |  | 68.0 |  |  | 52.0 |
Total reimbursements |  | $ | 221.4 |  |  | $ | 174.8 |  |  | $ | 157.3 |
Foreign Currency Transaction [Table Text Block] | The following represents the net gain (loss) related to impact of transaction gains and losses resulting from remeasurement for the years ended December 31, 2017, 2016 and 2015:
|  | (In Millions)
|  | 2017 |  | 2016 |  | 2015
Remeasurement of intercompany loans |  | $ | 16.6 |  |  | $ | (16.6 | ) |  | $ | 11.5 |
Remeasurement of cash and cash equivalents |  | (2.5 | ) |  | (1.0 | ) |  | 1.5 |
Other remeasurement |  | (2.7 | ) |  | 0.8 |  |  | 3.3 |
Net gain (loss) related to impact of transaction gains and losses resulting from remeasurement |  | $ | 11.4 |  |  | $ | (16.8 | ) |  | $ | 16.3 |
Adoption of New Standard [Table Text Block] | The following represents the estimated impact from the adoption of ASU No. 2017-07 for the year ended December 31, 2017:
|  | ($ in Millions)
|  | Year Ended December 31, 2017
|  |  |  | Estimate
Financial Statement Line Impacted |  | As Reported |  | Adoption of ASU No. 2017-07 |  | As Adjusted
Cost of goods sold and operating expenses |  | $ | (1,828.5 | ) |  | $ | 2.4 |  |  | $ | (1,826.1 | )
Selling, general and administrative expenses |  | $ | (105.8 | ) |  | $ | (7.7 | ) |  | $ | (113.5 | )
Miscellaneous - net |  | $ | 27.7 |  |  | $ | (1.7 | ) |  | $ | 26.0 |
Operating income |  | $ | 423.6 |  |  | $ | (7.0 | ) |  | $ | 416.6 |
Other non-operating income |  | $ | 3.2 |  |  | $ | 7.0 |  |  | $ | 10.2 |
Net Income |  | $ | 363.1 |  |  | $ | — |  |  | $ | 363.1 |

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