Fair Value - Nonrecurring Fair Value Measures (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Nonrecurring Fair Value Measured [Line Items]
Equity Method Investment, Carrying Amount	$ 124.5	$ 121.1
Goodwill, Carrying Amount	130.6	2.9
Intangible Assets, Carrying Amount	84.6	1.9
Remaining Net Book Value	3,669.1	3,284.8
Taconite Harbor [Member]
Nonrecurring Fair Value Measured [Line Items]
Remaining Net Book Value	100.0
Nonrecurring Fair Value Measurements [Member]
Nonrecurring Fair Value Measured [Line Items]
Equity Method Investment, Impairment		0.0
Goodwill, Carrying Amount	130.6	2.9
Goodwill, Impairment	0.0
Intangible Assets, Carrying Amount	84.6	1.9
Intangible Assets, Impairment	0.0
Property, Plant and Equipment, Impairment	0.0	0.0
Nonrecurring Fair Value Measurements [Member] \| Taconite Harbor [Member]
Nonrecurring Fair Value Measured [Line Items]
Remaining Net Book Value	$ 100.0
ATC [Member]
Nonrecurring Fair Value Measured [Line Items]
Ownership Percentage	8.00%
Equity Method Investment, Carrying Amount	$ 124.5	121.1	$ 114.6
ATC [Member] \| Nonrecurring Fair Value Measurements [Member]
Nonrecurring Fair Value Measured [Line Items]
Ownership Percentage	8.00%
Equity Method Investment, Carrying Amount	$ 124.5	$ 121.1
Equity Method Investment, Impairment	$ 0.0

This item represents an other than temporary decline in value that has been recognized against an investment accounted for under the equity method of accounting. The excess of the carrying amount over the fair value of the investment represents the amount of the write down which is or was reflected in earnings. The written down value is a new cost basis with the adjusted value of the investment becoming its new carrying value subject to the equity accounting method. Evidence of a loss in value might include, but would not necessarily be limited to, absence of an ability to recover the carrying amount of the investment or inability of the investee to sustain an earnings capacity which would justify the carrying amount of the investment.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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-SubTopic 10
-Section 35
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The percentage of ownership of common stock or equity participation in the investee accounted for under the equity method of accounting.

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This item represents the carrying amount on the entity's balance sheet of its investment in common stock of an equity method investee. This is not an indicator of the fair value of the investment, rather it is the initial cost adjusted for the entity's share of earnings and losses of the investee, adjusted for any distributions (dividends) and other than temporary impairment (OTTI) losses recognized.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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No definition available.

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Amount after accumulated impairment loss of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

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Amount of loss from the write-down of an asset representing the future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

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The amount of impairment loss recognized in the period resulting from the write-down of the carrying amount of an intangible asset (excluding goodwill) to fair value.

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The aggregate amount of write-downs for impairments recognized during the period for long lived assets held for use (including those held for disposal by means other than sale).

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Sum of the carrying amounts of all intangible assets, excluding goodwill, as of the balance sheet date, net of accumulated amortization and impairment charges.

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Amount after accumulated depreciation, depletion and amortization of physical assets used in the normal conduct of business to produce goods and services and not intended for resale. Examples include, but are not limited to, land, buildings, machinery and equipment, office equipment, and furniture and fixtures.

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